Also check out the Priority Boxes that are NOT flat rate. I was returning some merchandise to Holy Clothing. The medium flat rate was too small, and the large flat rate was too big. I had a weigh-based Priority Box that was the best size. I weighed it and the actual postage cost was LESS than the medium flat rate box. By $2.
And they also have regional rate boxes coming too. Or at least that is what the lady at the counter told me. I know that when I want to ship things back to Indiana, FedEx Ground is usually the cheapest.
On a side note - I HATE parcel post. It takes FOREVER regardless of the proximity of the addresses. I have had a parcel post box takes 17 working days from Newark NJ to Gatesville NC. And the post office could give me no reason for the long shipping time. The website said that the shipment would take 7-10 working days - uh right. If I purchase anything on-line (eBay or otherwise) I always ask if I can upgrade the shipping.