Build the Callout Service

public class PaymentResult {
    public Boolean success;
    public Boolean declined;
    public String gatewayTransactionId;
    public String message;
}

Three fields directly mirroring Lesson 1's three outcomes — success and declined together let a caller distinguish all three cases (success=true; success=false, declined=true; success=false, declined=false for an error), without needing a separate enum or extra parsing logic.

public class PaymentGatewayService {
    public PaymentResult charge(Decimal amount, String paymentToken) {
        HttpRequest request = new HttpRequest();
        request.setEndpoint('callout:Payment_Gateway/charge');
        request.setMethod('POST');
        request.setBody(JSON.serialize(new Map<String, Object>{
            'amount' => amount,
            'token' => paymentToken
        }));

        HttpResponse response = new Http().send(request);
        Map<String, Object> parsed = (Map<String, Object>) JSON.deserializeUntyped(response.getBody());

        PaymentResult result = new PaymentResult();
        if (response.getStatusCode() == 200) {
            result.success = true;
            result.gatewayTransactionId = (String) parsed.get('transactionId');
        } else if (response.getStatusCode() == 402) { // "Payment Required" — the gateway's decline status
            result.success = false;
            result.declined = true;
            result.message = (String) parsed.get('reason');
        } else {
            result.success = false;
            result.declined = false;
            result.message = 'Gateway error: ' + response.getStatusCode();
        }
        return result;
    }
}

Notice paymentToken, not a raw card number — Lesson 4 covers exactly why. The three-way branch here is a direct implementation of Lesson 1's success/decline/error distinction, using the gateway's actual status codes to tell them apart.