It took me quite a while before I switched from flat rate medium priority mail boxes to Regional Box A & Regional Box B because I didn't understand what they are, and you cannot get them at the post office.
Regional Box A - comes in two sizes - one about the size of the smaller 1095 USPS side loading priority mail box, the other a little smaller than priority mail medium flat rate top loading box. You can ship up to 15 pounds in these boxes, which is plenty for most sellers. Flat rate boxes are about $10-10.50 -- Regional boxes go by zone and are $4.97 - $9.97 for the farthest zones. That means for about the same as a flat rate envelope where you have to squish the item, you can often switch to a Regional Box A box. (Box B goes to 20 pounds) What I do is when I am ready to ship, I choose "calculate shipping" and it puts in all the options, including the regional boxes. If its close to the flat rate envelope, I go with that and my customers get a much less squished item in the mail.
Go ahead and order on-line - I usually order a box of 25 to be sure I don't run out, especially during busy seasons! I've received so many items that weight 2 pounds mailed to me in a flat rate envelope for $10.50 (and they usually charge me that amount) I want to say, "Hey, check out regional mail!" You won't be sorry!