What i do is to quote in the auction that item can be collected or posted using cheapest method i can reasonably find. i will then guess the cost of postage and normally double it etc. Once i have posted the item i include a cheque or refund using paypal the difference in postage.

If it's written in the ebay auction text if they bid they are accepting your terms and conditions and shoud be prepared to pay up to the amaount i have quoted but will probably get around half the postage cost back.

As i hate people that quote £10 to post something like a CD!