Send the buyer a message hopefully they'll understand the postage issue and will agree to a cancellation.
You can then cancel the order citing Buyer Requests to Cancel, this refunds the buyer, and your seller performance is not affected.
There are few choices to cancel without penalties, and yours unfortunately is not a valid one , a postage error, so if they do not agree whichever reason you select you may find yourself with a damaging defect on your account.
@178tadworth