Can someone explain the USPS prices? I know it says it on their website but it's confusing?

To figure out how much it would cost to ship something, go to usps.com and click on "calculate a price". Each screen will ask you several questions and give you several options. For Priority Mail that is NOT flat rate, ignore the pictures with Priority Mail boxes and go below where it says "OR - Select a shape". That is where you enter the shape and size of your box. It does not matter if you are using a USPS Priority Mail (NOT flat rate) box as those boxes cost the same to ship as any other regular box (the boxes themselves, however, are free from the Post Office).

As for weight over 13 oz. It depends on the weight and the destination. You would have to input that data into the form (as I noted above) to get that data.

It is ALL on the website in USA.