Hmmm, do you actually need to do them separately? In other words, could you not add a prompt before checking out with a digital item that says “do you need any add ons?” With a list of options so that they don’t complete the sale until you know they are already sure of all that they want? 

I don’t know the product/service they are buying exactly, so maybe this situation is a little different, but I feel I wouldn’t want someone to complete a sale for an item with add ons, until they have already selected all of the add ons — which sounds like what you are doing, but it seems you are trying to hack it together *after* the purchase, rather than ensuring the add ons have been fully selected *before* they actually make the purchase.

If you are having trouble (or honestly if you are having to code AT ALL in this situation — which is too much headache, imo — then I would go that route instead.

Unfortunately I don’t know enough about woocommerce or webdev stuff to be of much use if you are set on doing it via the “processing” route.

PS. Another reason I might stay away from hacked together code for an added “processing” stage, is because it’s likely to break very easily with any updates or changes to the other tools.