I think we're (http://www.pwinty.com) exactly what you're looking for. We're an API for printing photos and posters.
You call our API, we do the printing and shipping. All the billing, invoicing and checkout- you deal with.
Not doing mugs yet, but will do soon.
Get in touch via the website and I can send you a copy of the iOS library (its a bit rough round the edges, but should give you a good headstart).
- Do you also handle PDF or you prefer rasters?
- Are you considering handling payments directly from the customers?
Would be so much better w/ payment handling (one lib to integrate). We wouldn't even mind if the package is arriving with your logo.
We're not looking at handling the payments directly at the moment, as that could complicate things quite a bit (we have to store custom pricing for each merchant and each order etc).
It might be something we look at long-term, but not for now.
Is there a reason you only do rasters? I've believed that vector graphics is superior. The printer has problems with the rendering?