This is one of the biggest failing with Simple Delivery, you cannot combine postage after items have been purchased.
Has the buyer for them all? This is not simple, but the best way forward would be to cancel all items, of course, with the buyer's acceptance, and relisting them as one, so there's one important tracking number, one postage price. Ensure any message to your buyer makes it very clear the reason why the items are being cancelled, that they are to be relisted on eBay again, for the buyer to buy, leave no room for doubt that this is Not an off eBay sale.
Some sellers are leaving a message on their listing, for example..... ' If you wish to buy multiple items, please let me know before buying, I will relist them as one listing so only one postage cost is charged, combining postage is not currently possible. '
