See if you can send the buyer a combined invoice. It is for postage but puts all the items on one transaction. On seller hub go to the Orders tab > Awaiting payment. From the dropdown actions menu for one of the items select Send Invoice. If you can't do that you'll have to put the same tracking number on each one.
Either way it's still fine to use your own postage just remember to add the tracking and mark as dispatched manually via the dropdown actions menu for each item (or the one transaction if you manage to combine) on your orders page.