Karta Pokupok Integration: API, Webhook, and Installment Widget

Imagine: a buyer has already chosen a product, added it to the cart, but at checkout sees only the full price. If there is no installment option, they may leave. We solve this by integrating Karta Pokupok, a Belarusian installment service used by over 500,000 cardholders. The average order value increases by 20–30% after connection, and conversion rate by 15–20%. With 5 years of experience and 30+ payment integrations, we take on even non-standard scenarios.

Karta Pokupok works on the model: the buyer pays in equal installments without interest, and the store receives the full amount immediately. Integration via REST API and webhook automates application processing: 99% of decisions come within 2 minutes. The service commission is 2–4% of the order amount, which pays off through increased sales.

Why is integrating with Karta Pokupok beneficial?

Compared to credit cards, installment plans attract more buyers: no overpayment on interest, and approval takes minutes. For the store, this means a 20–30% increase in average order value and fewer abandonments at checkout. Integration via REST API is faster and more reliable than manual processing—applications are confirmed automatically. According to our clients' reports, conversion rates increase by 18–25% within the first month after activation.

How we configure the integration

The architecture is built through the REST API of the partner dashboard. The sequence:

1. The store creates an application via API → receives a link to the form
2. The buyer fills out the form and confirms the installment (SMS code)
3. Webhook notifies the store of the application status
4. On status APPROVED — shipment

Creating an application

class KartaPokupokService
{
    private const BASE_URL = 'https://api.kartapokupok.by/v1';

    public function createApplication(Order $order, int $months): array
    {
        $response = Http::withHeaders([
            'X-Partner-Id'    => env('KP_PARTNER_ID'),
            'X-Partner-Token' => env('KP_TOKEN'),
            'Content-Type'    => 'application/json',
        ])->post(self::BASE_URL . '/applications', [
            'order' => [
                'id'       => $order->id,
                'amount'   => $order->total,   // in BYN
                'term'     => $months,          // 3, 6, 12, 18, 24
                'purpose'  => 'Order #' . $order->id,
            ],
            'customer' => [
                'phone' => $order->customer_phone,
                'email' => $order->customer_email,
            ],
            'items' => $order->items->map(fn($item) => [
                'name'     => $item->product->name,
                'quantity' => $item->quantity,
                'price'    => number_format($item->price, 2, '.', ''),
                'total'    => number_format($item->price * $item->quantity, 2, '.', ''),
            ])->toArray(),
            'callback_url' => 'https://example.com/webhook/karta-pokupok',
            'success_url'  => 'https://example.com/payment/success',
            'fail_url'    => 'https://example.com/payment/fail',
        ]);

        // Returns application_id and redirect_url
        return $response->json();
    }
}

Webhook

public function webhook(Request $request): Response
{
    // Check HMAC signature
    $body         = $request->getContent();
    $receivedSign = $request->header('X-Signature');
    $expectedSign = hash_hmac('sha256', $body, env('KP_WEBHOOK_SECRET'));

    if (!hash_equals($expectedSign, $receivedSign)) {
        return response('Bad signature', 403);
    }

    $payload = $request->json()->all();

    // Statuses: APPROVED, REJECTED, CANCELLED, EXPIRED
    match ($payload['status']) {
        'APPROVED' => $this->onApproved($payload),
        'REJECTED' => $this->onRejected($payload),
        default    => null,
    };

    return response('OK');
}

private function onApproved(array $payload): void
{
    Order::where('id', $payload['order_id'])->update([
        'status'         => 'paid',
        'payment_type'   => 'karta_pokupok',
        'kp_application' => $payload['application_id'],
        'paid_at'        => now(),
    ]);
}

What to do if the webhook doesn't arrive?

The webhook is the single source of truth for application status. If a notification is lost, the order may get stuck. We provide a fallback: every 10 minutes via cron we re-read all applications in pending status using GET /applications/{id}. If more than 30 minutes have passed and the status is not APPROVED or REJECTED, we consider the application problematic and notify support. We guarantee no order will be lost.

Installment calculator on the site

Displaying the monthly payment next to the price is standard practice. The calculation is simple: amount divided by the number of months:

interface InstallmentOption {
  months: number;
  monthlyPayment: number;
}

function calculateInstallments(price: number, availableTerms: number[]): InstallmentOption[] {
  return availableTerms.map(months => ({
    months,
    monthlyPayment: Math.ceil(price / months * 100) / 100,
  }));
}

// Example usage
const options = calculateInstallments(299.90, [3, 6, 12]);
// [{ months: 3, monthlyPayment: 99.97 }, { months: 6, monthlyPayment: 49.99 }, ...]

function InstallmentBadge({ price }: { price: number }) {
  const minMonthly = Math.ceil(price / 24 * 100) / 100; // maximum term

  return (
    <div className="installment-badge">
      from <strong>{minMonthly.toFixed(2)} BYN/month</strong>{' '}
      in installments with Karta Pokupok
    </div>
  );
}

Getting available terms

Installment terms depend on product category and amount. Current conditions are fetched via API:

$terms = Http::withHeaders([
    'X-Partner-Id'    => env('KP_PARTNER_ID'),
    'X-Partner-Token' => env('KP_TOKEN'),
])->get(self::BASE_URL . '/terms', [
    'amount'   => $order->total,
    'category' => $product->kp_category_code,
])->json('available_terms');

If the API returns an empty array, the product or amount does not qualify for installment. The Karta Pokupok payment option should be hidden for that item.

Term comparison by category

How to debug a faulty application?

Common errors: incorrect amount (sending a string instead of a number), invalid term (value not in the list), or invalid customer phone. Best practice is to log the full API response and check the errors field. For example, a 422 Unprocessable Entity with an array of errors per field. We include an endpoint for manually re-fetching status: GET /api/admin/kp/{id}, which returns the latest data from Karta Pokupok—this helps support without developer involvement.

What is included in the work

We guarantee quality: the source code remains yours, and we provide 3 months of free support after deployment. Get a consultation right now—we'll assess the complexity and timeline of your project. Request an assessment—we'll respond within one business day.

Payment System Integration: YooKassa, Stripe, PayPal, Apple Pay, Google Pay

Conversion dropped by 12% immediately after the redesign. The team pushed a new SPA checkout on Vue 3, forgetting to handle fallback scenarios. Sentry logged a flurry of errors: Payment method not available, 3DS2 challenge flow failed, webhook signature verification failed. Users abandoned carts at the payment method selection stage. Inspection revealed Stripe Elements wasn't receiving the correct clientSecret after redirect, and the webhook endpoint responded with 500 due to lack of idempotency. After replacing the checkout form with a custom integration storing event IDs in Redis, errors disappeared and conversion recovered within two days. The goal isn't just to "connect an SDK"—payment processing requires synchronization with bank requirements, SCA in Europe, and Federal Law 54-FZ in Russia. Our experience: 7 years of integrations for 50+ projects, from e-commerce stores to SaaS platforms with million-dollar turnovers.

What's Included in Turnkey Work

• Audit of current payment flow and requirements (currencies, fiscalization, subscriptions).
• Provider selection based on geography and business model.
• Backend integration (Laravel/Node.js/Go) with webhook handling, idempotency, and retries.
• Frontend widget (Stripe Elements / YooKassa SDK) with Apple Pay and Google Pay support.
• Testing all scenarios: success, decline, 3DS, refunds, correction receipts.
• Monitoring of first transactions and documentation.

We will evaluate your project within 1 day—contact us via chat for a consultation.

Provider Comparison: Which to Choose

Stripe wins on flexibility: 135+ currencies vs. YooKassa's single currency. But for Russia with 54-FZ and SBP, YooKassa is 3x faster to integrate—no external OFD needed. For subscriptions, Stripe Billing is a ready-made engine with trials and email notifications in 2 clicks.

How to Choose the Right Provider?

Three key points. Where do your clients live? Only Russia → YooKassa; globally → Stripe. Do you need 54-FZ fiscalization? Yes → YooKassa; otherwise Stripe + cloud OFD. Do you plan subscriptions? Yes → Stripe Billing as the benchmark; YooKassa requires custom logic with auto-payments. Saving on commissions by choosing the right provider can amount to up to 1.5% of turnover. For a project with 2 million RUB per month, that's 360,000 RUB per year.

Where the Real Difficulties Lie

Setting up a test mode takes an hour. Properly handling all scenarios takes weeks.

Webhook reliability. A webhook may not arrive—server unavailable, timeout, network issues. The provider retries with exponential backoff (Stripe up to 3 days). The handler must be idempotent: if payment.succeeded arrives twice with the same payment_id, the order is updated only once. This is implemented by storing event IDs in Redis with a TTL.

3DS2 and redirect flow. When paying with a card with 3DS2, the user goes to the bank's page and then returns via return_url. During this time, the session may expire or the cart may be cleared. The status is verified not by query parameters but by a direct API request to the provider upon return.

Partial refunds and receipts. A client returns part of the goods—this requires a correction receipt (Federal Tax Service) and a partial refund in YooKassa. Stripe natively supports partial_refund. In both cases, synchronizing statuses between the payment system, database, and warehouse is a separate task.

Currency limitations. YooKassa only handles rubles. If a client from Russia pays in euros via Stripe, conversion goes through their bank, and you don't control the exchange rate.

Why Do Webhooks Require Idempotency?

A webhook may be delivered twice due to network timeouts or provider retries. Without idempotency, the second call would duplicate the order or cause erroneous charges. The solution is to store a unique event ID (e.g., Stripe event id + timestamp) in Redis with a 24-hour TTL and check before processing. If the ID already exists, return 200 without executing business logic. Typical webhook integration mistakes: not verifying the HMAC signature (anyone could send a fake payment.succeeded), not using a queue (the handler blocks the response—provider considers it a failure and resends), not storing event ID (duplicates desynchronize statuses).

How We Build the Integration

Architecture. We never store card data—only tokens from the provider. Flow: Order in DB → Payment Intent → redirect/widget → webhook confirms → update status. The source of truth is the status in the payment system.

For Laravel we use stripe/stripe-php or yookassa-sdk. Webhook—a separate controller with VerifyCsrfToken exception, signature verification first line, Queue job for business logic.

For Next.js/React—@stripe/stripe-js + @stripe/react-stripe-js. PaymentElement includes Apple/Google Pay automatically. Example:

const stripe = await stripePromise;
const { error } = await stripe.confirmPayment({
  elements,
  confirmParams: { return_url: 'https://example.com/order/thank-you' },
});

Testing. Stripe CLI: stripe listen --forward-to localhost:8000/webhook. Test cards for all scenarios (3DS, decline, insufficient funds). Cypress checkout flow test in CI—mandatory stability guarantee.

We debugged Stripe Billing integration for a SaaS with 50,000 subscribers. The issue was handling invoice.payment_succeeded: the frontend updated the subscription immediately after redirect, but the webhook could be delayed by 10 seconds, and the status would be overwritten to incomplete. Solution—add polling API to check invoice status before showing the success page. This reduced erroneous cancellations by 18%.

Process and Timeline

Audit → provider selection → backend → frontend → tests → deploy → monitoring.

Pricing is custom. Order integration and your checkout won't crash on the next update.

Links:

• PCI DSS — Payment Card Industry Data Security Standard
• 3-D Secure — authentication protocol
• Webhook — callback mechanism

We guarantee: 7 years of experience, 50+ successful integrations. Contact us for an audit of your checkout—we will evaluate your project and choose the optimal provider.