You do NOT have to buy the boxes for USPS. They are FREE and you pay for them when you ship the box. I know because I have shipped USPS hundreds of times. I have a stack of USPS boxes that I ordered and I paid $0 for.
Just did some research and you are infact right. I got ripped off at the ****ing shipping store.
XPZ, Alum heads,150#, I/C, GEN V mps 3.0, N* TB CAI K&N fully built trans, JE pistons, L32 rods, Comp lifters, PRJ Fuel rails, 65# Injectors, elec pump, MSD wires, MSD coils, Alum control arms, AGX, vogtland, GMPP sway bars, rear trailing arms