Zachin L'Adam Shelo B'fanav

The halacha is that if something is considered a zechus for someone we can employ the rule of zachin l'adam shelo befanav. Meaning, that if something is considered to be beneficial for someone, I can act for that person without being appointed a shliach. I can do it even without his knowledge.

What exactly is considered to be a zechus though?

The Ktzos HaChoshen (195: 2,3) discusses this issue. The Ktzos brings a machlokes haposkim regarding a matanah. What if I pick up a matanah on someone's behalf. Some hold that this is obviously a benefit. However, others hold, based on the pasuk of Sonei Matanos Yichyeh, that this may not be considered a zechus.

The Ktzos also discusses the possibility of zechus by a sale. What if there is a real good deal and I want to buy the item for someone and be zocheh the purchase for him? Can we say that this is a zchus? The Ktzos wants to suggest that this would be a valid zechiyah. See there for the full discussion.