Simple answer with Simple Delivery there is no facility to send a combined invoice, OR to safely send more than one item in a parcel.
One way round is to cancel sales and relist as one item, then tracking covers both items. Time consuming and risk of buyer giving up and not buying again.
Otherwise solution is to refund overpaid postage, and then risk sending two items in one package,BUT tracking will only prove delivery of one, and buyer could open a not received case to get a refund.
Wonderful system that only serves to make money for ebay, and leaves the problems to sellers!!
