I recently sold an item which I had quoted a pre-paid stachel for postage. The satchel is a cheaper option than boxing, and it can be packed and sent safely without any damage. But the buyer has since messaged me after winning the auction and asked me to post in a box with padding. This will most likely end up costing me an extra $5-$10 on top of what I had specified for postage. Am I allowed to ask for extra to cover the new postage? If I have to fork out the extra postage myself, and after Ebay fees, I'm almost giving the item away.

I'm not a big seller, so I'm not sure of how to approach the situation. I don't want to make money off postage, but I shouldn't have to pay the extra if the buyer requests a different postage method without contacing me before bidding, right?