Say you need to pay someone 10mil
The person who needs to be paid sets up shells/fruit/whatever in their re-tail and sets the price for 999,999 bells each.
The person paying comes in and buys the shell/fruit/whatever for the set price. For 10mil it needs to be done 10 times. 20mil, 20 times.
I think it's really only effective if their re-tail is near the train station, personally. If it's far away it's probably easier to just drop the money bags on the ground.