Smashing the piggybank
You can only break it open once.
Organizations (and political candidates) that forget this and treat their biggest supporters like bottomless ATMs learn the piggybank rule at great cost.
Every interaction you have with a customer either strengthens your relationship (because it’s mutually beneficial) or weakens it. Weaken it enough time and you break it.
The fact that you ended up with a few bucks in the meantime is immaterial compared to the long term damage of breaking the relationship because you’re in a hurry.