If the postage was stated in the listing then wait 4 days and open a non payer dispute and 4 days later close and give them a well deserved non payer strike and get your fees back.
If you are going to charge more than the advertised postage then send them a request to mutually cancel.
Both these options are to the right of the sold item from the drop down select resolve a problem and follow the prompts for the one you want too do.
