First class shipping is usually $2.51 but what the buyer doesn't realize is that sellers also add in cost for packaging and paypal fees, which is what you probably did here. And I would just explain to them that your fees include shipping and packaging and the cost was laid out in the auction and if they won the auction, they were agreeing to pay the cost mentioned. If they give you less than a positive because of your shipping cost, eBay should have your back and you'll be able to get that removed.