The default postage for subsequent items is buggy when you use the bulk-edit feature. For example, if you select 10 items for bulk-editing, then change them to $8.95 first item and $0 subsequent items, ebay will calculate it correctly. However, if you then click on Revise Item on one of those changed in bulk-edit (individually, by opening the item and revise), the subsequent item postage has $8.95 in the box. So if you were editing quantity or description and did not look at the postage and proceeded to save the changes - voilla! - you are now charging $8.95 for subsequent items.

Back to the OP's issue, though, it is very simple: if it is an error in the listing, you have to either cancel the transaction or suck it up. I would never send a message to the buyer for more money. Just put yourself in the buyer's shoes, you bought something and the seller asks for more money for whatever reason, you wouldn't be too happy either.