Favor Box is included among Recommended Products for Kid Birthday Party Suites. I don’t understand why not for Adults too, I suppose it was an involuntary omission, not intentional.
A major disadvantage of not having it as a Recommended Product is that the other Favor Box variants (e.g. other box shapes) in the same cross-selling collection don’t appear under it as “Other styles for this product”. And the design transfers very badly between two different favor box shapes (e.g. between tent and gable).
Adult favor boxes sell pretty well AFAIK. At the time of writing, on the search page for “birthday favor box”, the 2nd product is an adult birthday favor box for the 90th birthday:
https://www.zazzle.com/s/birthday+favor+box
Edit: in my own experience:
My most sold adult birthday favor box has 1734 views and 22 orders:
My most sold kid birthday favor box has only 149 views and 5 orders:
So in my own experience, adult birthday favor boxes are more successful than kid ones and I see no reason to not include them among recommended products, but include the others.