New install – no donation option in cart

First, perhaps I have the wrong understanding of the add-ons functionality, but thought it would make it so that there was always a donate option available in the cart no matter the product.

I’ve installed/activated the add-on, created my donation product, which is visible in the store and works as expected, but I can’t seem to get a donation option to appear in the cart or checkout. I’ve tried several themes but none display any fields related to making a donation.

The page I need help with: [log in to see the link]