I learned an economic secret from my grocer

From India, a grocer mathematician.  I saw that he had done the 1.99, 2.94, 3.99 stuff with the prices.  I commented, and he quizzed me, I suggested  a reason, I got it wrong.

When he delivers a nickel and penny back to the customer, that formally seals the deal; the customer accepts the change, he can't complain later. And this is a better use of small change, when the alternative is packing it up in rolls.   The change acts like a speed bump for every transaction.

I get to name this the gateway law, the name of the store.  But the store is meant to be just that, a gateway, you transact, in and out, then continue on.  I guess the road was an oldie trail at some point.