I have been shipping for my online businesses for 6 years. I have always found that any package over 5 pound and NOT within the same state as the shipper, UPS and/or FedEx is cheapest. Lately FedEx has been beating UPS...but you just have to check both against the other when you ship.
I see something about a "regular" flat rate box...guess that means they are now going to use the $ from price hikes to have ANOTHER size box manufactured
But if you use usps online shipping you will save some on all priority shipping (5% I think) over going to the post office. And no line plus you can schedule a pickup at your house so you never have to step foot in the post office. A decent digital scale that goes up to 5 or so pounds is pretty inexpensive. Any thing over that I just use an old kitchen scale 25# max and I have never had a problem. Just print the label out on a regular sheet of paper, cut it out and use some packing tape to stick it on, put it on the front porch (after scheduling a pick-up) and you are done.