I've never used the QR code option, but I've read that it can be hit-or-miss at the post office. Sometimes they'll get an error code that says the label is not available. The Priority Mail long rectangular MED/LARGE flat rate boxes are self-sealing (the square ones are NOT self-sealing), but I usually add a strip of tape over each flap to be safe. They have packing tape for sale at the post office, or he can ask them to throw a strip of the Priority Mail tape over the flaps (that tape is free to use when you're mailing via Priority).
One important note: before you sell an item that will be shipped from another location, you need to make sure that the eBay listing shows that the item ships from HIS location (not your zip code), and the label you purchase will need to show HIS address for the shipper's address (not yours). It's flat rate so it won't affect the postage, but the post office can refuse the package if the origin address is out-of-state.