Prices, Products, and Subscriptions can each be made discountable, allowing any coupons entered on the form to affect their price. When this setting is toggled off for a particular item, that item will be full-price even if the user has entered a coupon code for a percentage discount. All fields related to payment are discountable by default.
Configure a question to toggle the "Discountable" setting on or off. For Products and Subscriptions, each item has its own toggle.
If coupons are not enabled yet on the form, the "Discountable" toggle won't appear when configuring your Products. Head to Configure → Payments → Coupons to enable them first.