It depends on where you live and where you are shipping to. Because I live in the Northeast and ship a lot to the northeast, regional boxes are the best way to go. Not all post offices are happy to use them though, because they are techincally intended for business use. (I do sell on ebay, so I guess that works).
The other way is to just check priority mail shipping without the flat rate. If boxes are really heavy or are being shipped across country the flat rate is the best deal. If it isn't particularly heavy or if you are just shipping several s tates away.....regular priority mail is usually a better deal.
Honestly, I would check with your local PO.