This explains the order of operations. Discounts rarely apply automatically; you often need to add items to your cart in a specific sequence.
Some combos involve sites like G2A or Kinguin. These are "gray market" resellers. Keys from these sites are often bought with stolen credit cards. If a key is revoked, Steam support will not help you. Stick to authorized vendors: Humble, Fanatical, Green Man Gaming, and Gamesplanet. steam combo list exclusive
: Never use the same password for Steam that you use for your email, social media, or other gaming platforms. This explains the order of operations