Can anyone explain why this happens? Someone buys two items from me, then pays for them, but eBay doesn't combine the purchase so I end up paying the Paypal transaction fee twice.

It never happens when I buy stuff - eBay always combines purchases from the same buyer so I only checkout once.

It just happened today with someone buying the SAME ITEM twice, then paying a few minutes later - but in two transactions. Why doesn't eBay combine the purchases?

(eBay customer service has given a couple of irrelevant answers when I queried them about it, eventually blaming Paypal.)