If your buyer has already paid it's too late to amend their invoice. All you can do is refund in cash when delivering. Remember that eBay will still charge full fees for the sale, including the fees on p&p.
It doesn't look like you've sold anything value lately, but bear in mind that without proof of postal delivery all your buyer has to do is claim not to have received it - eBay would enforce a full refund. Including the postage cost you have already returned to them!
