I basically use the API to create a checkout specific to the user's order, then listen to the webhook to fulfill the credits. Gateways are Shopify and PayPal. Shopify payments are in my bank account in a couple of days.
Do you need a PayPal account and dabble with their API to set up this type of solution?
Or is a Shopfiy account enough?
However, if you're wanting to sell digital products, it may not be the best option for you. I've had to do a bit of extra work to get it to work for my situation.
Aren't your prints also like digital goods? I would expect you do not print them yourself but forward the image and order data to some whitelabel printing solution?