Right thats what I am saying, I use the flat rate large box and stick the tradesy label on it, which means it is not technically flat rate postage. A few times I actually took boxes this way to the post office at the counter and nobody said anything, I had no idea I was doing anything wrong.Unless packages are heavy and going across the country, the FRB prices are higher than non-flat rate. While I can't recall reading stories here (on TPF), there are cases that have been posted on the ebay boards where buyers get hit with postage due costs because sellers used FRB but paid the non-FRB price.
I'll bet you anything that the post office actually prefers you use their boxes vs various odd size ones, they probably have a loosey goosey rule about it. I'm not suggesting anybody else do this, though.