You can remove the ability for these misunderstandings/shenanigans (they could be either) by setting up Immediate Payment Required (IPR) on all of your fixed price listings.
I set this up a few months before the Christmas season a few years ago, because at Christmas, like most sellers, I was getting a higher percentage of non-payers and losing out on the chance to sell my item to other Christmas shoppers, and it worked so well I have never looked back.
For the present issues, tell the buyers you cannot issue invoices for GSP transactions and recommend that either they call eBay so that eBay can help them with their account issue, or you will be glad to file a mutual cancellation if they wish. Once you have them on record as asking for the cancellation, it won't count against your defect rating to grant it.
The $60 shipping buyer probably didn't use the cart, but purchased each item separately - you would then be required to ship each item separately to KY. If he requests a cancellation, you can cancel, put everything in one listing, and let him buy the one listing -- that will give him one GSP shipping charge which will be far less than $60.