At the moment, across the UK site, sellers are not able to send invoices at all and can't send invoices combining postage.
If you can't contact the seller, you can either not pay and risk unpaid item strikes or pay and hope that the seller will send the items together and refund your excess postage.
If the seller is using Ebay's new Simple Delivery, the money you pay for postage will go directly to Ebay and not be paid to the seller at all so sellers may not be in a position to give refunds/discounts for multiple purchases.
It is all a bit of a mess, but not your seller's fault.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)