The difference is simple:

A "KOO-pon" will get you a maximum of 50 cents off whatever is being sold.
A "Q-pon" has no limit to its worth.

The same rule applies to "vase" (rhyming with "face") defined as a jar in which to stick flowers, compared with "vase" (pronounced "vaahz") meaning something from the Ming dynasty which, when you see it on a museum floor, you immediately pick up any two-year-olds you may have brought with you.

By way of illustration, there was a men's clothing store (somewhere in the states, I forget where) who offered a Q-pon worth $100 off a new suit. The kicker was that this Q-pon was posted on a highway billboard and anyone having the moxie to "clip" it would not only be risking life and limb but would also be breaking the law as well. Don't know if they ever got any takers.