Have you sold an item, is that it? Then you can send the buyer a revised invoice. The following link will tell you all about it:
http://pages.ebay.ca/help/pay/checkout-invoices.html#invoice
When you do it, do not use 'Recalculate'. That does not work and takes you back to square one. Just make any changes necessary, then Preview, then Send.
If you want to revise an active listing, this tells about that (heh):
http://pages.ebay.ca/help/sell/edit_listing.html
and the next one here shows all the things you can and cannot change, depending on whether there are bids, how much time left, that sort of thing:
http://pages.ebay.ca/help/sell/revising_restrictions.html