, unless you have a contracted account that they only charge you by the weight
Yeah, I usually only check by weight unless it is a weird size. Don't do that much styrofoam shipping though.
USPS had those flat rate boxes that you can make work. They get pissed when you cut them up and modify the box though. Try to charge you a different rate. I told them it is the same cubic volume.
They have no sense of humor.