For business cards, stickers, greeting cards, labels, etc Moo has an API http://us.moo.com/api/

I haven't used the API but I've bought business cards from Moo and they were great.

Thank you so much. The API does seem quite complex (it looks like you need to go through dozens of steps to print). I wish they could take Stripe's approach and simplify it somewhat. This seems like a great opportunity for someone to disrupt this market.

infraprint.com just started one about two weeks ago. I haven't had the opportunity to test it yet but it looks like it has all the features that you might need.

I'm planning to use them to print and mail custom letters through the USPS for a credit repair site I'm building.

For credit repair, you'll need to do certified mail for certain requests. This is something we're getting ready to launch :-) Feel free to reach out (info in profile), if we can help.

This looks promising. Thank you!

I've been working on something like this with the printer I use at http://cheergram.com.

There are a few hurdles like dealing with large files and handling them when out of spec, but its definitely a good idea.

Hi there, Founder of TryPaper.com here, we handle some of the core printing & mailing processes all via API :)