https://partner.steamgames.com/doc/store/application/bundles

Overview

There are two types of Bundles to handle common scenarios:

1) Complete The Set - Customers that purchase this type of Bundle will pay for, and receive, just the items in that bundle that they don't already have in their Steam library. This is most useful for bundles such as "Collector Pack" or "Franchise Pack" or "Developer Pack" that include two or more products (including games, software, or DLC). The customer can purchase one of these bundles as long as there is one or more item in the bundle they do not already have in their account. These types of Steam Bundles can only be purchased for the account making the transaction and cannot be sent or stored as gifts.

2) Must Purchase Together - This is useful for bundles such as "Soundtrack Edition" or "Deluxe Edition" that include just a base game and single piece of DLC or related application (such as a Soundtrack). The customer can purchase this bundle only if they do not already have any of the products in the bundle. These types of Steam Bundles may be purchased as gifts.

Originally posted by Nx Machina:

https://partner.steamgames.com/doc/store/application/bundles

Overview

There are two types of Bundles to handle common scenarios:

1) Complete The Set - Customers that purchase this type of Bundle will pay for, and receive, just the items in that bundle that they don't already have in their Steam library. This is most useful for bundles such as "Collector Pack" or "Franchise Pack" or "Developer Pack" that include two or more products (including games, software, or DLC). The customer can purchase one of these bundles as long as there is one or more item in the bundle they do not already have in their account. These types of Steam Bundles can only be purchased for the account making the transaction and cannot be sent or stored as gifts.

2) Must Purchase Together - This is useful for bundles such as "Soundtrack Edition" or "Deluxe Edition" that include just a base game and single piece of DLC or related application (such as a Soundtrack). The customer can purchase this bundle only if they do not already have any of the products in the bundle. These types of Steam Bundles may be purchased as gifts.

I still dont understand why the Price changes AFTER the Bundle being added to the cart, with it becoming much more expensive than advertised

Im experiencing this issue right now during the fall sale. I want to purchase FBC firebreak deluxe edition, I already own Control Ultimate edition, so the bundle with the 2 shows as $26.98 for me, but when I add it to the cart it jumps up to $30.57. What the hell is going on?