I had a buyer with the same problem last week.  She said the invoice was correct in the email but when she clicked on Pay Now it only showed one item instead of both.  I assumed she was clicking on Pay Now in the email but she may have been doing that from her purchases section, but just checked that the emailed invoice was correct.  She said she was using her mobile but didn't say whether she was using the app or not, but in the end she managed to pay through her cart (not much help if your buyer didn't use the cart to buy).