both involve small requests, followed by larger requests. the difference is in completing vs. committing to the small requests.
foot-in-the-door involves you asking parents for $5, they give you the money, then you ask for 10 more; or internet companies ask you for 1-min survey, once completed they direct you to a 20-min survey --- in other words, the persuader makes you complete an easy, small request before making larger requests.
lowballing involves you going to a car dealership, the salesman convinces you to buy a car for 10 grand (so now you are committed to buying it, but haven't bought it yet), he then talks to his manager and tells you 10 grand is a bit low but the car is all yours for 1 grand more --- in other words, persuader makes you commit to a smaller, easier request (without completing it) before making larger requests
