Every time this has happened to other sellers, the buyer had used a promo code to make the purchase, and eBay did recently have that 15% off deal, so I can almost guarantee that's what's probably causing it. 

I used the promo code when purchasing 3 items from a seller the other day. They were all free post, so combining wasn't a consideration (the seller sends everything separately, anyway, as the package sizes are quite large) and I just added each one to my cart then checked out.

For some unknown reason, though, my payment was split into two, so I check out once, but the seller would have received two separate payments, and been charged for the privilege (I spent $150, so they're unlikely to be concerned about the 30c extra PayPal nabbed, but it's not the first time that's happened, and it's been on much lower value items, too, so if it happens enough times, PP would be making a fair bit in extra fees).