You can send a revised invoice but you can only amend the postage price, so if there are 3 postage costs, that is the maximum you can deduct, if there is any more to deduct, that will have to be a partial refund made through Paypal.
Remember you will pay full eBay fees on the original listed price, eBay do not deal in partial refunds.
Go right of the item sold to More Actions and select Send Payment Details. All items will appear on the one invoice. Amend the postage amount to that required, but do not click on reset. Check it is correct, and submit.
