Upload & Sell: Off
I want to this myself -- use Costco. I haven't found anything that provides the tight integration to anything other than Blurb. But I thought of using LR4 Book module to create my pages, then export each page to a jpeg to the dimensions of my book (in my case 12" x 12"), then import each page to Costco. I did one page and it seem to import properly, though I have printed anything yet.
Just a thought anyway.