If these were all purchased separately, and you are now posting them together, because there is a tech issue with combining postage there is no way to mark the items as dispatched , especially if you did not use eBay's Special Delivery label, then none of the items can be marked as dispatched , and you'll have to wait a minimum of 14 days for those payouts to be released.
I'm presuming the items have already been posted?
If not, then the best way forward would be to cancel all items, of course, with the buyer's acceptance, and explaining the reason. Relisting them as one item, 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.
