Print on demand services:
Mixam – Has a print on demand service called Print Link (available in the US and UK). Unfortunately, this service does not allow for integrations to feed into it. Also, they got a bit frustrating when they replaced a paper we had clearly specified with another stock and didn't notify us..and then refused to provide any sort of even partial refund of other concession. So...beware.
Mixam has no ShipStation integration. From my observations, there is also no bulk order uploading that would allow you to put in different shipping addresses as part of the same order. Very expensive per piece price.
Lulu Direct – Connects with Shopify. You can import orders into Shopify for print on demand with Lulu.
Blurb – More limited conventional book sizes (8x10, 12x12), not our first choice.
Pirate Ship