Listing shows $15 for First Class, if you can't revise the shipping amount when making payment and/or the seller doesn't send a revised invoice then tell them to take a hike!
While they are out hiking report them under the eBay Non-Performing Seller policy.
http://pages.ebay.ca/help/policies/seller-non-performance.html

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.