Some NPCs have three dots connected by lines above their heads and in the menu on the map. This means that you can establish friendship with them and get bonuses for it
Friendship with the NPC sways as for the glasses work and for the gifts
Every NPC has different interests, and above his head indicated the gifts, the giving that you will receive 10 points of friendship if you will give something else of their list you will obtain 2 times less friendship points
A list of all the NPCs with which it is possible to pump friendship, and the reward you get for different levels of relations can be viewed through the main menu by clicking on the button with 3 dots connected by lines.
One example of a good relationship with the NPC, a 5% discount on its product