I have had buyers do the the same thing. Recently, I had a buyer whom purchased four of my listings separately and paid the full shipping for each which I actually don't mind. I don't cancel. Instead I just refund the difference less the transaction fees, and just add the tracking number to all orders. It's actually better for you because if they need to return an item then it won't affect of the other purchases if they request a return from one of the listings.
For example, I purchased two of the same electronic computer items from a seller. It was advertised to be compatible with both laptop, and desktop computers. Unfortunately, they weren't compatible with desktops only laptops. When I went to return one eBay automatically opened the return claim for both devices. There's no work around for the seller. Either the buyer returns both or negotiates a partial refund
