I started with an API called Pwinty which turned out to be pretty expensive so I ended up basically recruiting a print shop partner based in the US.
I've actually been looking at releasing a 4"x6" photo printing API of my own since we've developed some pretty neat shipping tricks as well as the ability to print on both sides of the photos.
I was actually looking at it from a different business model perspective. There are some APIs that do both print and ship in various formats but there is definitely a markup on top. Aggregating printers that are digital enough is quite a challenge of its own though :)
I've actually been looking at releasing a 4"x6" photo printing API of my own since we've developed some pretty neat shipping tricks as well as the ability to print on both sides of the photos.