Medium and heed the warnings about taping the crap out of it, double boxing etc,. I had a medium box arrive with 18#'s of what started out at 70#. I think the postal workers get mad when they get a heavy box and do everything they can to try and get the contents to spill out.
I'm glad that I'm not the only one who has that suspicion about the postal workers and heavy flat-rate boxes.
