Apparently, whoever set the price at the soda machine at school to 85 cents never had to do the stamps problem.
People are typically going to pay with either a dollar or exact change.
If you pay with a dollar, the machine needs a dime and a nickel to make change.
If you're paying with exact change, it's probably with 3 quarters and dime. (Even if you happened to have two nickels in your pocket, most people are likely to use the least number of coins possible).
Quarters are rarely given as change, so we quickly fill up and the machine can't accept quarters. Because you are constantly giving out more nickels in change than you're typically bringing in, you run out of nickels and thus can't accept just a dollar.
Buying a soda becomes a puzzle, trying to find a combination of singles and coins that equals 85 cents without using any quarters and without requiring any nickels in the change. And if you don't have any dimes or nickels, you're screwed.
All for a damned soda.