Not sure about payment gateways, but if you're using a decent shopping cart system, it will offer the functionality you are seeking. Most shopping cart systems now-a-days offer the ability for the shopper to enter a coupon or discount code to redeem discount on the final checkout price.
Beyond Google Checkout, which platforms let merchants issue and redeem coupon codes at checkout?
What is 3D Secure, how does it protect transactions, can it increase checkout drop-offs, and are there alternatives?1 ansWhat do customers typically expect from an online payment gateway?1 ansAre there payment platforms that support coupons and discount codes during checkout besides Google Checkout?1 ansWhat is SEPA Direct Debit, and why is it considered challenging for e-commerce use?1 ans