I am referring to a buyer who buys several items and THEN pays, not someone who buys and pays for each one in turn. eBay customer service had similar reading comprehension difficulties when I asked them about this.

Considering that some on here had the same problems as eBay, maybe it's you with the comprehension problem..

Maybe you need to re-read some of your OP

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?

This reads to me as the buyer bought two things from you and paying for them separately.