There are a couple of modest resources for 
shipping - one of which is a thread from Riven's
board: http://pub20.ezboard.com/fanimecelsbuytrade.showMessage?topicID=48.topic

There is also an illustrated guide by someone, 
unfortunately I don't remember who ^^;;;; (it was 
brought up in #anime-cel... hmm...) Surely someone
will remember it and post it here ..

And yes, Priority stuff is free - you can even 
order it bulk on-line from their site, which I 
have done. I personally use the smaller rectangle
boxes, flattened, cut the nonadhesive flaps off,
fold the adhesive flaps over, and tape the cut off
flaps to the back of the package - small, little 
waste, and efficient. ^^