Get him to put them in the cart and click 'Request an Invoice'.
If that doesn't work, as it possibly won't, set up a single listing for items he wants, with the correct postage.
I do it as a BIN with best offer enabled, and set the BIN way higher than the agreed price. That stops gazumpers. The buyer then makes me an offer for the agreed price, I accept, they pay, I post. It's a bit of mucking around, but if eBay's system won't allow the buyer to request an invoice, it is probably the easiest workaround.
