I thought I'd start this thread while America is sleeping. I've only included companies that I've bought from personally, so feel free to add. (I've also included some retailers of ultralight gear.)
Companies that offer USPS 1st Class Postage as a standard option:
Peter's Head Nets,
Companies that don't offer USPS 1st Class Postage as a standard option (but may if you email):
HMG (I've had things shipped 1st class after contacting them),
I thought about adding information about which companies offer help with customs ;) but decided that might be unwise on an open forum.