The 33 1/3 second mystery #14
 

Oscar Mudge, a traveling salesman, was a man of habit. During 1997, wherever he was in the US, he could effect the same purchase of $3.84 with a $5.00 bill and receive back four coins in change every time. What did Oscar buy?

some stamps from an automated machine? And gets $1, 10c, 5c, 1c

cliche claims another fuzzy bunny. Good work for someone across the pond!

Um.... I know I'm very late to this one, but it seems to me the amount of change you receive has absolutely nothing to do with (a) how often you go, and (b) what you are buying.

Whatever you buy for 3.84 in a vending machine is going to give you 4 coins in change, assuming the use of dollar coins

Seems to me the answer could have been "a pack of cigarettes" or "a sandwich" or one of about a million other things you might find in a vending machine.

Which leads me to ask the following: how the hell did you know it was a book of stamps?

Well, he was anywhere in the US, which means that the price is fixed and isn't taxed. Not many choices for that.

also i think they started using dollar coins again in '97. something like that.

Also very few vending machines will give you pennies.