You can create a custom coupon code for your customer by going to Settings --> Coupons, or clicking here: https://app.gotphoto.com/config_sell/index_coupons
You can (but do not need to) set a minimum order value, and it's important to limit the number of uses to "1" so that customers can use the coupon only once.
The customer can enter the code in the cart and the amount of cart value will automatically be reduced by the amount in the coupon.
Please note the max percentage is 50% for coupons.
Also, please be aware that coupons cannot be combined in the store.