This kind of thing (multiple purchases but payment for only one) happens to me quite frequently, usually the buyer responds and confirms the extra purchases were an error of some kind and I can get the transactions cancelled quickly enough, but recently one buyer didn't respond. I had the same concern re: opening a UPI on the unpaid item, but at the same time I wasn't going to let that mean I have to pay eBay money for nothing. I won't risk sending a cancellation request, either, not without the buyer being 100% sure what's going on and their confirmation they'll agree, as a couple of times they thought I was trying to cancel the item they paid for and it took me a little while to explain that they had purchased 2, paid for 1, and the unpaid one is what I wanted to cancel.
So, my first message just asked them whether the second purchase was in error etc, then a few days later I sent one further enquiry, explaining that any orders which remain unpaid will automatically go through the process to void the transaction and recover fees, and I would not be able to delay that process unless a cancellation was requested before it commenced. I figure if I explain the situation and give ample opportunity for their input, they can hardly blame me when what I said would happen, happens.
They didn't respond to that either, so a few days after that, UPI opened and then closed without any reply - no FB for the paid-for item yet, either (purchase was early November).
