Postage Costs

Can someone confirm for me what sort of difference there should be between the cost of posting a package within your own country compared to sending it overseas? By my understanding, all the packaging would probably be the same, which just means a few dollars extra to cover the additional postage costs. Is this right? Or are there further costs of which I'm not aware?

Yep, this is an ebay thing. If it only costs US$5 to post a small graphic novel within the US, I don't see how it costs US$21 to post it internationally.