Most of my Zen Cart mods don’t update the cart contents – there are only two exceptions.
Auto Add, which has the ability to add an item when another item is added using the product info page. It is sometimes used for free items or items discounted using one of my other Zen Cart discounting mods.
Recently a customer asked if he could do a one-time-only auto add for a product which was commonly paired with another product, so I updated the API to permit this behavior.
(Can you guess the other exception? It’s Coupon Auto Add!)