If it was your last 3 items sold and to the same buyer then the ebay item sold email would say $9 and they may have paid using that before you sent the invoice.

You cannot ask for more than $9 if that is what you quoted unless the buyer asks for a different delivery method and then you need to refund the payment and reinvoice with the new postage and don't click recalculate, if you do it goes back to $9.