I paid $10.00 postage for an ebay item and the postage was actually $3.63?

You can e-mail them with your comment and see if they respond. This is typical of most businesses & e-bay sending products etc. They give an estimated amount based on weight, packing materials and handling fee (time+gas spent getting to post offfice etc.) to cover all their expenses. If you accepted the amount from your bid with shipping, then that's what you pay regardless. Sorry!