That's alright, as long as the auctions don't have any bids on them you can still edit their descriptions. If there is a bid, you can leave a comment on your own auction detailing the shipping information.
The auctions listed page is probably the best way to go about this now (http://www.listia.com/account/auctions_listed). On this page is a list of all the auctions you have listed, and you'll be able to edit them one at a time.
You'll need to include shipping information to every auction that's "buyer pays shipping" as soon as possible to avoid them being removed for not enough information.