Photo Products -> Print Credits
Print credits allow you to offer prints and / or downloads to your customers at no cost.
Basically a print credit is a collection/package so you will need to create a collection first for what is included in the print / download credit in Photo Products - Collections.
When creating a print credit, you select the collection you want included. These collections don't have to be part of any price list.
There is a redeem code created the customer will use to redeem the print credit. Then they can select the photos for their print credit and checkout at no cost unless there are other products or shipping applied.
Enable the Redeem Print Credit link in the menu
Go to Site Design > Menu Links and edit that link and make it enabled so your customers can enter in the redeem code.