Forum Moderators: buckworks
If they aren't on-board with your reasoning, they may (1) do a chargeback for any over-amount (regardless of your shipping policy, you may lose), or (2) simply take their business elsewhere, leaving you holding a returned package.
Customer mistakes are a cost of doing business, chargebacks are a tool they can use effectively, and unless the party agrees to pay for the over-charges, you need to eat the losses and move on.