It is something I have done for buyers in the past. But I have never done it when postage is more than quoted. And not without prior communication.

There is no longer an option to mark as sent when pickup is selected. I archive them.

As far as postage goes, you have done your dough.

In future, if somebody plays that trick cancel using problem with buyer's address. They will get their money back, you will be refunded your fees.

Then block them. If they can't contact you prior to the sale, they are trying to game you. It's the 80/20 rule.