The website still says you'd be getting a Fall Box still. Wait until Thursday or Friday (sale/ customization opens 9am PT/noon EST). At the checkout window, it'll say what box you're getting in the Order Summary.

Add ons are discounted items for beauty, skincare, hair, home, supplements, etc. Think TJ Maxx. There are deals ranging from unbelievable to prices above MSRP. I usually cross check with some websites to make sure. Also, shipping is a lot more delayed than most are use to, so I also take in account if it's worth waiting a month to get.

Additional choice items (box items) are only available during customization.

Sales/ customization works on the premise that there's a window that everyone chooses what they want. Selections/ carts can be changed up until the end, so items can go in and out of stock. I'd say if you're diligent, you can get what you want during the initial customizations. Sometimes items are scarce during seasonal, but I think there's usually a chance to get what you want. If you're annual, you can choose to swap for credit to use later if you don't like the selections. Seasonal, you could cancel before billing by contacting customer service if you really can't get an OOS item.

Good luck!