Thank you for the reply, Smelly.Corner.
I did try to purchase/gift the add-on from the store directly, both via the Horizon app as well as the webpage. But no luck.
Digging a little deeper, the root of the issue appears to be how the Family Center treats the purchase of in-game currency, by calling it an "add-on". We have run into this issue with both Animal Company and now with Sail.
When attempting to purchase in-game currency (for cosmetics, etc,.for example) the Family Center appears to treat this as if trying to purchase a DLC pack. Which typically would have a proper storefront listing on the Meta store.
That would explain the dead link in my email, since in-game currency packs do not show up as anything searchable or purchasable from within the Store app itself.
So it creates this infinite error loop where I cannot purchase the currency pack from the store, because there is not store listing, and trying to purchase it from the headset just restarts the loop by sending me another email to allow the purchase.
Not sure if there is a solution path given the inner workings of the Family Center.
Kinda frustrating.