@Rob
At this stage of the auction, you are unable to revise anything, so the winner is entitled to pay only the postage you set when the ad was posted.
The rule is no revisions if there are bids or no bids and less than 12 hours to go.
You could well lose out on this one!
[EDIT: Ignore. I see you did so already! Still up yon tree I'm afraid.]
JaX
