Done by underage males, a guilt free beer run is an average beer run except that the person committing said beer run slams down exact change or more than the cost of the beer on his way out. This means he will not have to show his ID yet doesn't feel guilty to the clerk for "stealing." This is a prime example of a situation in which everyone wins as the underage male gets his beer and the store clerk is paid the money owed but cannot be reasonably held responsible for allowing a minor to purchase said alcoholic commodity.
18 year old Doug: Here have a beer, we did a beer run earlier.
18 year old James: You're a dick! The poor attendant is going to have to pay for the missing merchandise!
18 year old Doug: Nah don't worry bro, it was a guilt-free beer run; I slammed down correct change on the counter as I ran out!
18 year old James: Oh ok, pass me a cold one then!