I found http://printaura.com via Google when we first decided to wire the generator up to a t-shirt print service, then I saw a http://shirts.io post on HN and wired them up too!
When each order is placed, we go with the printer that gives us the largest margin. There is no bidding going on or anything, they just have different prices for different products and calculate shipping fees differently.
Started as a joke among my dev team and we've now sold a few shirts and plan on developing the idea.