This issue can sometimes happen when the QR code is generated using mobile data, rather than Wi-Fi. Try connecting to a stable Wi-Fi network and generating the QR code again. You can also try restarting the app, and then selecting the QR code right at the post office.

If this doesn't work, you are able to purchase your own USPS Media Mail label and enter the new tracking number into the order information. To input your new tracking number, just navigate to the sale under "My Sold Books" in the account tab. Then, scroll to the bottom and you'll see an "Add My Own Tracking Number" option. This will void the original label and release the funds used to pay for the original label with your earnings.