buying the SAME ITEM twice, then paying a few minutes later - but in two transactions

There were two transactions and they did pay separately - that's the point of my complaint. I said:

Someone buys two items from me, then pays for them

The "then" is the key word - they buy everything first, then pay for everything.

I only offer one postage service, so that's not it. Buyers could be choosing to split them but I don't know why they would - I haven't asked (yet).

A possible explanation is that eBay's computers are just too slow when the first purchase is a long time before the second. I haven't kept records because I didn't realize this would keep happening, but twice recently buyers have bought the first item hours before the second, then paid in two transactions because eBay didn't combine them. Maybe the purchase history needs time to refresh/sync them up?