With the new add-ons app, sellers are able to currently offer one-time products as add-ons to their subscription products on their storefront website. This only adds the one-time product add-on to the first shipment.
I'd be nice to offer an add-on product that gets added to each shipment and charge for that add-on being in each shipment.
So if they choose a monthly term then every time it renewed, they would also get charged for the add-on product and it would get added to their shipment. If they chose a 12-month prepay then it would be in every shipment and the customer would pay for that upfront (add-on price x 12).
Barkbox does this by offering to add an extra premium toy to each box during the checkout flow