After I have selected the wrong container and need to add more postage, you have to recreate a new label edit the shipping container and procedure is to cancel the first shipment and request a refund, wait to get reimbursed which causes delay and extra processing and time for your dept and mine.
If you allowed two options:
Ability to create a postage label in the amount needed to cover the shortage if we need to add additional postage to cover the cost. (Stamps.com has this feature and it is the only thing that I miss!
Ability to print stamps for first class letter stamps or labels without having to pay the Thick Envelope rate 3/4 or Package rate at a higher rate to allow printing ounces for first class under 16 oz.