It is an insert but I'd say that wherever TCDB files the 80s Topps Company Store advertisement/offer cards is where this should go under 1990 Pro Set.
For those who weren't around/interested in Pro Set history, this card was an SP rotated into packs at the same rate as the others: Payne Stewart, Santa, etc. So while it is definitely an ad card, it qualifies an insert as well.
It is safe to say that it is NOT a promo card at least in the sense that it wasn't issued ahead of the product's release to preview a product, outside of packs, at shows, to dealers, or any of the other ways promo cards are distributed.