We use regional rate boxes a LOT for our businesses that ship. USPS doesn't make it easy though...you have to search the rate for everything to find which is best. In order to get the best answer on the calculator, we need to NOT check the flat rate box, but enter a weight THEN see what comes up for Priority. Sometimes it's a flat rate padded envelope (for coffee), sometimes it's a regional box, other times it's best to use the non-flat rate priority boxes and then sometimes NOT using their box at all is best rate. It's maddening.FLAT Rates are only good occasional. I rarely use them. Usually cheaper to order box #7 and pay by the weight.
Despite what you read regional rate boxes are TOO EXPENSIVE to use. I have never seen a time the rate was cheaper with regional rate. If you have a smart phone you can get the USPS app and that will help you pick the right box. Weigh it and check rates by your zip and the zip you are shipping too. It will give you prices for dropping off at the post office or printing postage on line and prices for each option like Express, flat rates, regional boxes, and reg boxes.