Chuck is walking down the street and someone asks him to sign a petition to put clean-air legislation on the next ballot. he decides to sign the petition. then the person asks whether he would like to give a donation to the associated environmental group.
this technique is known as

foot in the door

Explanation:

Foot in the door is a persuasion technique in which a small request is made knowing that if a person agrees to a small request it is likely that a person won't refuse when a bigger request is made.

You can think of this as when you have your foot in a door it is likely that the owner of the house won't shut the door once the foot is in.

Having people sign a small petition is a small request that no one can refuse which opens a way to the bigger request to ask for donations. After people have signed the petition it is likely that they won't refuse giving out the donation.