While waiting in line at the post office, I noticed several stacks of empty boxes under the counter. I asked about them and the clerk told me that they are the flat rate shipping boxes.
The way it works is different from normal postage requirements. With normal boxes the amount of postage that you have to pay is determined by the weight of the box and its contents. But with the flat rate shipping, they have boxes of different sizes that you take home (or back to the office) and pack as much as will fit into the box. The weight does not matter unless it is so heavy the box breaks. The postage you pay is by the box, not by the pound.
The small box flat rate is $11 and the larger box fat rate is $14. These go “priority” so they get delivered within 3 days to most places in the U.S. That’s a pretty good deal except for books. You can still send books much cheaper using the media rate.