The US Postal Service recently added some new boxes and envelopes to their Proirity Mail line-up. I primarily use Priority Mail to ship my eBay and Amazon packages.
They are now offering a Regional Rate box, well 2 of them actually. These boxes provide the speed of Priority Mail while allowing you to ship heavier items at a lower cost, similar to the flat rate boxes.
You have 2 choices; Regional A box which has a 15 pound limit. The Regional B box has a 20 pound limit.
The cost to ship is a flat rate based on region. Like regular Priority Mail, it will cost more to ship from one coast to the other, and less within your own region.
Here is the cost to ship using these boxes: