Thank you for the suggestion. I've set it up as you suggested as I don't need them to manually input an amount. Now I just need to figure out how to get rid of the shipping and return text from the product page. 

Hello,  We have a physical restaurant and are looking to use Shopify to sell on-demand printed T-shirts and a few other things. I'd like to also be able to sell physical gift cards to our restaurant as well. These would be cards paid for on Shopify, ...