Recording a payment method is an important step when creating sales forms, and it helps you realize how your customers like to pay you. You're definitely able to record a payment method on a Sales Receipt in QuickBooks Online. I'll explain where to look.
When you open the Sales Receipt creation screen, the Payment method drop-down will be on the left side, above the line items. You can select a payment method from the list, or click + Add new to create a new method of payment.