You can setup a Coupon Discount for your customers from the Store / Discounts section of your admin account..
1) Hit Store
2) Click on Discounts
3) Now hit the + Coupon Discount button to the top right.
4) Now work your way through the form fields by first entering a Name for the coupon discount and a Coupon Code.
When entering a Coupon Code, take note that this code is globally unique for your shopping cart. It can be used in any gallery and for any image. Once a Code is set it cannot be reused. Try to also use a coding system that isn't easy to predict by website visitors.
When entering the Discount Amount you have the option to use a percentage based discount (e.g. 20% off) or fixed dollar amount discount (e.g. 20+ for $20 off).
If you would like to create single-use coupon discounts, make sure to set Number of Uses to 1.
The Active From and Expiry Date settings can also be used here if this coupon discount is only usable for a certain window of time.
5) When you're done setting up the Coupon Discount settings, hit Create and you're done!
IMPORTANT: It is possible to set the Discount Amount field to 100% off if you wish.
However you need to be aware that you (as the photographer) would then be footing the bill for the wholesale Print and Shipping costs when these would normally be passed on to your customer.
If you're running a test order for you own purposes though, then this is the best way to make that happen!