Make It Asynchronous (Queueable)

public class ProcessPaymentJob implements Queueable {
    private Id orderId;
    private Decimal amount;
    private String paymentToken;

    public ProcessPaymentJob(Id orderId, Decimal amount, String paymentToken) {
        this.orderId = orderId;
        this.amount = amount;
        this.paymentToken = paymentToken;
    }

    public void execute(QueueableContext context) {
        PaymentResult result = new PaymentGatewayService().charge(amount, paymentToken);

        Order__c order = new Order__c(Id = orderId);
        if (result.success) {
            order.Status__c = 'Paid';
            order.Payment_Reference__c = result.gatewayTransactionId;
        } else if (result.declined) {
            order.Status__c = 'Payment Declined';
            order.Decline_Reason__c = result.message;
        } else {
            order.Status__c = 'Payment Error';
        }
        update order;
    }
}

This is Module 38's Queueable pattern doing genuinely real work: execute() calls the synchronous-looking PaymentGatewayService (which is fine now, since it's running in its own async transaction, not inside the original trigger), then updates the Order__c based on Lesson 2's typed result.

public class OrderTriggerHandler {
    public void afterUpdate(List<Order__c> newOrders, Map<Id, Order__c> oldMap) {
        for (Order__c order : newOrders) {
            Order__c oldOrder = oldMap.get(order.Id);
            Boolean justReadyForPayment = order.Status__c == 'Ready for Payment' && oldOrder.Status__c != 'Ready for Payment';

            if (justReadyForPayment) {
                System.enqueueJob(new ProcessPaymentJob(order.Id, order.Total__c, order.Payment_Token__c));
            }
        }
    }
}

This is Module 24's Trigger.oldMap transition-detection pattern (from Module 26's rollup) combined with Module 38's System.enqueueJob — the trigger itself never makes a callout at all, it only ever enqueues asynchronous work.