eBay: Buyer changing postage price after purchase

Hey everyone,

My partner made an eBay purchase, her first ever (she registered just to get my Christmas present). She paid for the transaction which obviously included the postage price. Today she got an email from the seller saying that he went to the post office and postage came to nearly double what he thought it would, and he wants her to cover the balance.

This seems pretty outrageous to me. Are there any policies on eBay that govern this sort of thing, either allowing it or outlawing it?

I would read through the T's and C's myself but this is obviously quite a time-sensitive issue.

Cheers.