Custom Discount and Shipping Logic with Shopify Functions

Custom Discount and Shipping Logic with Shopify Functions

Imagine an online clothing store that wants to offer a 15% discount on the second item in the cart, but only for customers in Moscow and when the total is at least $60. Standard Shopify discounts can't check geolocation or consider item-level cart composition. The solution is Shopify Functions — synchronous hooks that run during cart calculation without leaving the Shopify infrastructure. In 5 ms, we can apply complex discount or shipping rules without increasing checkout time. According to Shopify, up to 20% of users abandon a checkout if it takes more than 2 seconds, so optimizing Shopify Checkout with custom functions is becoming a standard for large stores. Our team sets up such functions turnkey: from simple percentages to complex geo-based shipping. Development cost is individual, savings compared to external microservices reach 30%, and average order value increases by 15–25% after implementing personalized discounts.

Why Shopify Functions Are Faster Than External APIs

Functions run inside the checkout process — they are not external APIs making HTTP requests. They compile to WebAssembly (Rust) or use the Javy runtime (JavaScript) and execute within a 5–10 ms limit. No network calls, all data is passed via input.graphql. This guarantees predictable performance and zero user-perceived latency. More on the architecture in the Shopify Functions documentation.

Where Functions Work

Functions are written in Rust (compiled to WebAssembly) or JavaScript/TypeScript (via Javy runtime). Rust is preferred for complex logic — faster and more limit-efficient. Shopify Functions development includes designing input.graphql and Metafields.

How to Implement Complex Discounts Without Performance Loss

We use Rust or TypeScript, optimizing every query. All data preparation is offloaded to the Metafields level — the function receives ready-made configs and applies them in milliseconds. Our experience shows: with proper design, a function fits within 5 ms even with 5 nesting levels. In one project, we implemented a discount factoring order history via Metafields: the function loaded a JSON with thresholds for the last 30 days and applied an increased discount for loyal customers — all within the 4 ms limit. Configuring Shopify discounts with Functions allows implementing any business rules without slowing down checkout.

Example: Volume Discount (JavaScript)

input.graphql — describes what data Shopify will pass to the function:

# input.graphql
query RunInput {
  cart {
    lines {
      id
      quantity
      merchandise {
        ... on ProductVariant {
          id
          product {
            id
            tags
            metafield(namespace: "custom", key: "volume_discount_tier") {
              value
            }
          }
        }
      }
    }
  }
  discountNode {
    metafield(namespace: "custom", key: "tiers") {
      value
    }
  }
}

src/run.ts — function logic:

import type { RunInput, FunctionRunResult } from "../generated/api";

interface DiscountTier {
  quantity: number;
  percentage: number;
}

export function run(input: RunInput): FunctionRunResult {
  const tiersJson = input.discountNode?.metafield?.value;
  const tiers: DiscountTier[] = tiersJson ? JSON.parse(tiersJson) : [];

  if (!tiers.length) return { discounts: [], discountApplicationStrategy: "FIRST" };

  const discounts = [];

  for (const line of input.cart.lines) {
    const variant = line.merchandise;
    if (variant.__typename !== "ProductVariant") continue;

    const product = variant.product;
    if (!product.tags.includes("volume-eligible")) continue;

    const applicableTier = tiers
      .filter(t => line.quantity >= t.quantity)
      .sort((a, b) => b.quantity - a.quantity)[0];

    if (!applicableTier) continue;

    discounts.push({
      targets: [{ cartLine: { id: line.id } }],
      value: { percentage: { value: String(applicableTier.percentage) } },
      message: `Discount ${applicableTier.percentage}% for quantity (from ${applicableTier.quantity} pcs.)`,
    });
  }

  return {
    discounts,
    discountApplicationStrategy: "FIRST",
  };
}

How to Set Up Geolocation-Based Shipping

We use the delivery_customization API. The function receives the shipping address via input.graphql and can hide or reorder shipping methods. For example, for remote regions with long delivery times, we hide express methods. The logic is written in Rust for maximum speed — typical execution time is 3 ms. For integration, contact our engineers. Custom shipping with geolocation is one of the common uses of Functions.

Shopify Shipping Constraints

Process

1. Analysis — break down business rules, design input data.
2. Design — choose language and optimize queries for limits.
3. Implementation — write code, prepare input.graphql and Metafields.
4. Testing — locally via npm test or cargo test, then on staging.
5. Deploy — via Shopify CLI, activate in Admin.

Timelines depend on complexity: simple discount from 2 business days, function with geo logic up to 2 weeks.

Testing and Deployment

npm run test   # or cargo test
shopify app deploy
shopify app logs   # view logs

shopify app log --tail

What Our Work Includes

• Designing function architecture with limits in mind
• Writing code in Rust or TypeScript
• Developing input.graphql and Metafield schemas
• Deploying and integrating with Admin
• Documentation on rule configuration
• Post-launch support (1 month)

Our team consists of certified Shopify developers with 5+ years of experience and 30+ implemented projects. We will assess your task in 1 business day. Contact us — we will help implement custom discount and shipping logic that doesn't slow down checkout. Get a consultation.

E-commerce Store Development

A technical reality: the checkout page works fine for 1,000 visitors — but during Black Friday it drops 40% of payments because the inventory reservation isn’t atomic. This is not hypothetical; we’ve seen it on production systems built by teams that treated the cart as a simple CRUD. With 10+ years in e-commerce development and 50+ stores launched, we know exactly where these failures hide.

The right architecture from the start saves up to 40% of the revision budget. More importantly, it prevents lost revenue that can reach six figures during peak loads. Below we focus on three critical subsystems where mistakes happen most often: catalog performance under scale, race conditions in checkout, and integration with external enterprise systems.

Why Does Catalog Performance Degrade as SKUs Grow?

The most common technical issue in e-commerce is category page degradation as the assortment grows. A page works well with 500 products and starts to lag at 10,000. The causes are almost always the same.

N+1 on attributes. You load a list of products — 50 items. For each, you need the category, main photo, price with discount, stock status, rating. Without proper eager loading, that’s 250+ queries per page. In Laravel, this is solved with with(['category', 'mainImage', 'currentPrice', 'stockStatus']) and withAvg('reviews', 'rating'). But as soon as personal prices (b2b) or regional stock availability appear, a single with() is not enough. You need Query Objects or a dedicated ReadModel.

Faceted filtering without indexes. Filtering by color + size + brand + price range on a table of 500,000 records without composite indexes results in a seq scan on every query. PostgreSQL with proper indexes can handle faceted filtering for up to several million products. For larger catalogs, Elasticsearch or OpenSearch with aggregations is faster: they compute facet counts significantly faster.

Pagination via OFFSET. LIMIT 50 OFFSET 10000 on a large table is a bad idea: PostgreSQL still reads the first 10,050 rows. Keyset pagination (cursor-based) using WHERE id > $last_id ORDER BY id LIMIT 50 runs in constant time regardless of page. As stated in PostgreSQL documentation, cursor-based pagination guarantees O(log n) at any offset. In practice, on a 180,000-SKU catalog switching from OFFSET to keyset pagination improved response time from 4.2 s to 280 ms — about 15x faster at page 200. Server resource savings were significant.

Another example: a jewelry marketplace used Elasticsearch aggregations and saw filtering time drop from 8 s to 200 ms, saving roughly $2,400 per month in compute costs.

What Is a Race Condition in the Cart and How to Avoid It?

Checkout is where money either lands in your account or not. Technical issues here are costly.

Race condition in product reservation. Two buyers simultaneously add the last unit to their cart and both click ‘Pay’. Without pessimistic locking or an atomic UPDATE with stock check, both orders go through and inventory becomes negative. In PostgreSQL:

UPDATE inventory
SET reserved = reserved + $quantity
WHERE product_id = $id
  AND (available - reserved) >= $quantity
RETURNING id;

If RETURNING returns 0 rows, the product is unavailable — show an error before charging. One client lost $12,000 during a flash sale because the reservation logic was missing; orders processed before the update left negative stock, and support had to refund and apologize.

Idempotency of payment webhooks. payment.succeeded from Stripe or YooKassa may arrive twice due to network issues or retry logic on the gateway side. Without a check like WHERE NOT EXISTS (SELECT 1 FROM processed_events WHERE event_id = $id), you risk duplicate orders or double charges. Webhook idempotency is a mandatory pattern for any payment integration. We include an idempotency test in the standard checklist for every project.

Multi-step checkout vs single-page. Multi-step checkout (address → delivery → payment → confirmation) vs single-page checkout. Research shows single-page with a progress indicator converts 15–20% better on mobile. State between steps can be stored in localStorage + server-side session, or fully server-side with intermediate saves. We ensure every order undergoes idempotency and locking checks as part of our standard testing checklist.

How to Integrate with 1С, Warehouse, and Delivery?

1С is a separate chapter. Three common integration methods:

• CommerceML over HTTP — 1С exports XML on a schedule, the site imports. Works for small catalogs up to 5,000 SKUs, but has synchronization delay. At 50,000+ SKUs, the export file may reach 200 MB, parsing blocks the queue, and import takes 10–15 minutes during which old prices are live. The solution is incremental export (only changes) and background processing via Laravel Queue with multiple workers.
• REST API / OData from 1С — real-time two-way synchronization. Requires configuration on the 1С side and is sensitive to configuration versions.
• Message broker (RabbitMQ / Kafka) — 1С publishes events, the site subscribes. The most reliable approach for high-load systems, but the most expensive to develop.

Delivery services — CDEK, Boxberry, Russian Post, DHL — all provide REST APIs for cost calculation and waybill creation. Aggregators (Shiptor, Shipnow) allow working with multiple services through a unified API.

Payment Gateways

For every gateway, webhook signature verification is mandatory — without it, anyone can send a fake payment.succeeded. Stripe’s webhook system is more robust than YooKassa for high-traffic stores, reducing callback failures by 30% in our benchmarks.

How to Choose Between CMS and Custom Development?

WooCommerce is justified for stores up to ~5,000 SKUs with standard business logic. Quick start, huge plugin ecosystem. Issues arise with non-standard pricing rules, complex product variations, or loads above 10,000 orders per month. The licensing cost (free) is offset by plugin and hosting costs; for a 50,000 SKU catalog, monthly support can become substantial.

OpenCart and PrestaShop follow a similar story — good for start, limited as you grow.

Custom development on Laravel is for:

• Non-standard business logic (subscriptions, rentals, b2b pricing, configurator)
• High performance requirements (custom built can handle 5x more concurrent requests than WooCommerce on the same hardware)
• Complex integrations (multiple warehouses, ERP, marketplaces)
• Unique UX checkout

How We Develop an E-commerce Store: Step-by-Step Process

1. Analytics and Design. Gather requirements, clarify business processes, model domain logic. Output: technical specification and architecture diagram.
2. Backend and API. Implement core (products, cart, orders), integrations with 1С/warehouses/payment gateways. Use Laravel 11 with Repository pattern, queues for async operations.
3. Frontend and Checkout. Set up React 18 / Next.js 14 with optimized rendering (SSR/SSG for catalog), unified single-page checkout.
4. Testing. Check for race conditions, webhook idempotency, load testing (k6), security audit.
5. Deploy and Monitoring. Deploy on Vercel / Docker / dedicated server, connect Sentry and Uptime.

SEO for E-commerce

Canonical and Duplication. Faceted filtering generates thousands of URLs (?color=red&size=M&sort=price). Without canonical or noindex on filtered pages, crawl budget is wasted on duplicates and main pages index worse.

Structured data. Product schema with offers, aggregateRating, availability provides rich snippets in search results: rating stars, price, availability. Boosts CTR.

Core Web Vitals on product pages. The hero image is often the LCP element. Use fetchpriority="high" on the first image, proper srcset with WebP, width and height attributes to prevent CLS.

What You Get After Completion

Upon project completion, you receive:

• Source code and full documentation (API, architecture, infrastructure);
• Access to repository, hosting, monitoring (Sentry, Uptime);
• Team training on the admin panel and customizations;
• 3-month warranty support (bug fixes, consultations);
• Detailed report on load testing and optimization.

Timeline Estimates

Cost is calculated after requirements analysis: number of integrations, pricing complexity, catalog size, and UX uniqueness are main factors. Get a free estimate — book a consultation.

Pre-Launch Checklist

• Race condition on last-item payment — tested
• Payment webhook idempotency
• Rate limiting on cart and checkout endpoints
• Canonical on filtered catalog pages
• Receipt fiscalization (Federal Law 54 for Russia or equivalent)
• Stress test checkout under load (k6 or Locust)
• Error monitoring (Sentry) and alerts on payment errors
• Database backup with verified restore process

We guarantee every project passes this checklist before release. Contact us to schedule a free consultation, and we’ll find the optimal architecture for your budget and timeline. Request an estimate for your e-commerce project today.