The folks behind UnitedPixelworkers.com give away all the behind the scenes dirt on selling and shipping a whole bunch of t-shirts to nerds everywhere:

We’re not Threadless, but having grown two orders of magnitude since starting this little union, we know the pain of scaling in every part of our operation. What’s the difference you ask? Everything.

They cover switching from Big Cartel to Shopify, sourcing a printer, and many other decisions (and mistakes) they've made along the way.