I would normally agree with you, but I won the bid at the lowest price available. That means this person couldn't sell the painting that week. If they relist it they could make more, plus they say on their site if you don't pay be a certain time, then the will relist it. They lose nothing by me not paying them, if they sell it again later.
The thing is, when I sell something online, I charge the exact shipping cost plus the cost of the box. thatis it, I don't charge $20 dollars for handling, thats crap, I wrote them an email asking about their handling fees, if they give a reasonable answer, then I will pay