# Gateways

# Custom Gateway Listener

# Your REST API

When customers select your custom gateway on ATShop and proceed to check out with it, their browser will send a background (XHR) request to the URL configured in your Custom Gateway settings. Alongside some query parameters identifying the order they are trying to pay for.

Since the requests are issued directly by customer's browsers and not from our servers, it is important that your API has the proper CORS settings.

Let's imagine your configured gateway URL is https://api.example.com/payment-link.

When a customer tries to check out with your gateway, their request will look similar to the following.

GET https://api.example.com/payment-link?orderId=4wuvZe7XgCZapx746 

Notice how orderId=... is appended to the URI. This is important to identify the customer's order. You can query our API using this ID to retrieve the customer's email address and how much they are expected to pay for their order.

To redirect customers to a custom checkout page, return a JSON response in the following format;

{
  "type": "url",
  "url": "https://example.com/checkout/abc123"
}

# Calculating payment amounts.

Retrieve the order's product ID and multiply the product's value by the order quantity. Orders that are discounted will have a toPay property attached.

const orderId = request.query.orderId;
const order = await HTTP.get(`https://i-api.atshop.io/shop/orders/${orderId}`);
const product = await HTTP.get(`https://i-api.atshop.io/shop/products/${order.productId}`);

/**
 * An integer in US cents representing how much the customer should pay for this order. 
 */
const value = order.toPay || Math.round(order.quantity * product.value);

The above example uses our experimental REST API for the sake of simplicity. It is however recommended that you utilize the DDP API to retrieve this data.

• Retrieve an order
• Retrieve a product

# Fulfilling orders

To fulfill an order, refer to our order documentation.