Since I'm too lazy right now to go check, are there still limits on websites so you can't buy stuff if you have a PO Box?
PO Boxes and buying online
I don't think there's ever been a law about it. It's just that a PO Box is no good for official service of legal writs if the person who buys doesn't pay, so most commercial websites are very wary of them.
Some places won't ship to PO Boxes and some will. I don't think there's an acutal limit.