I accept that from next month that if buyers don't wait for a combined invoice, and pay for items separately, then they will sent those items separately, regardless of how much they could save if only they had waited for a combined invoice. I very much doubt however, that buyers will ever 'learn' to wait for combined invoices.

But as for the current problem - the default invoice that ebay presented contained both items (each costing $7.50), plus the total of their individual postage costs (2 x $8 = $16.00). I am quite sure that the buyer would be quite happy to get the two items in the one satchel and pay just $8.00 postage instead of the insane situation where the postage for each item cost more than the item itself.

But this hasn't answered my original question - is there a penalty if I give a partial refund of $8 to the buyer?