The usual way would be to refund the postage. That would get your fees back.

If you are not going to do that, mark the item as sent, wear the fees on postage, and hope your buyer is honest. Without proof of postage you open yourself to an Item Not Received (INR) claim.

I don't believe you can change the postage method post-sale.

I trust my buyers, so I would refund and collect the cash money for the additional item on pick up. I have done it several times in the past.