This is terribly frustrating as many things were against you from the start here.
It looks like the seller had an unpaid item case selected on their listings so if payment was not made in ** days this would automatically open. They must have forgotten to deselect this if they agreed to wait for a second purchase to be made.
The second issue lies, and it's not a known thing amongst all sellers, that is, it's currently not possible to combine postage, sending that revised invoice. Items do have to be paid for individually, and a seller then refunds the excess. Not ideal, but that is how it works for the time being.
All I can suggest and I know you've contacted Customer Services before but contact them around 8 a.m. on a weekday, there's more chance of Dublin answering explain the above, the seller agreed to wait for 2 items to be purchased, but their auto selected unpaid item case led to you receiving those strikes.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
As said, I recommend contacting CS early in the morning as there's more chance of Dublin answering.
@rksj2005