Where do my customers enter in their discount code?
For direct payment gateways, a discount code can be entered during the checkout process, while the customer is asked to select a shipping method and enter in their payment details.
PayPal Express is an offsite gateway and works differently. PayPal Express will send your customer to PayPal.com in order for them to log in and authorize the payment. Once completed, the customer returns to your checkout page, where they can enter their shipping information, discounts, and complete the sale.