foot-in-the-door technique
Door-in-the-face technique
Bait-and-switch
Ben Franklin effect
Camel's nose
Creeping normality
Drip pricing
First they came ...
If You Give a Mouse a Cookie
Low-ball
Obedience
(human behavior)
Salami tactics
Selling technique
Stone Soup
https://en.wikipedia.org/wiki/Foot-in-the-door_technique