send them some money back via paypal?
I feel really bad about it. Calculated the cost on line and it came to £24.00 but took it to the post office yesterday and it was £12.00. The price of postage is on the label they put on package so buyer will know how much it cost.
Shall I just send them £10.00 today and a message explaining or wait until they recieve it first?? I don't want to look like I was trying to rip them off.