There are discounts for different types of mail already, though. (bulk mail, for example.)
Currently you get to choose which type of mail you're sending, you get to choose your discount. It's not about the discounts. It's about the carrier making the decision which one you get, and taking that choice away from you.
"What if your mail carrier had sole power to decide how quickly (or if at all) you got your mail?"
It's probably not a particularly accessible analogy for most of us here, but it's easily relatable across generations without getting bogged down in technical details.