You can only amend the postage on the invoice, before a payment is made. Then, go right of the item More Actions >> Send Payment Details, amend the postage, do not click on reset. Check it is correct and send.
If the buyer has paid, and you wish to offer a discount now, you have to send a partial refund through Paypal, but be aware you will still pay full eBay and Paypal fees on the original listed price.
