If you are the seller, you can send a revised invoice, but you can only change the postage amount, not the sale price amount. If the postage is too high, insert the new lower postage amount and DO NOT hit the recalculate button as that changes it back to the original amount. Once you've put the new amount in, send the invoice. As mentioned, that will override any previous invoices sent.
If the listed postage price is lower than what it should be, check with the buyer first that they are happy to pay the extra as they don't have to pay more than what was in the listing. It's up to the seller to know how much their items cost to post.
