If somebody buys several items from me and pays the various separate postage, I refund the difference less eBay and (often) Paypal flagfall.

If they ask prior, I set up a separate listing for what they want, with a ridiculous BIN and Best Offer enabled. They offer the agreed price, I accept, they pay.

I don't know that I've lost sales doing that as I get no more than one query a month and seem to be refunding overpaid postage to those that don't enquire at least once a week.