Sellers can't send combined invoices these days. And combining isn't working properly with Simple Delivery although eBay have said
"If your buyer has purchased more than one item from you in a single transaction you’ll receive one prepaid label to post them all together. If your buyer has purchased items from you in different transactions you’ll receive postage labels to post each item separately."
The best thing to do to save postage for your buyer is to cancel the transaction with buyer's agreement and relist the items all one listing with one postage for them to buy again.