Ok, well ... sometimes gas places sell car washes and discount the gas by 10 cents a gallon. So, they probably wouldn't refund if that was the case.
If you have a few minutes, you should look up "caveat emptor." What were the terms of sale? Does Shell have a refund or credit policy? You think they're being unreasonable, but technically you purchased a car wash at a certain rate, but since you did it at the pump instead of at the car wash, there was not a reasonable expectation of immediate service.
If the cashier was being an asshole, you need to document it and inform his manager. The most appropriate and professional way of doing so is waiting for his shift to complete and then calling and asking how you submit a comment to a manager.
If it truly becomes your life mission to take down the Shell corporation of a $6 car wash, that you can redeem at any Shell station with a car wash in the United States - or at your local Shell station once their system is up and running again - you truly lead a sad, sad life. I suggest you seek counseling or grow a set of balls, whichever fits within your budget. And, since $6 seems to be the point at which you lose rationality, I'd suggest you just spend a half hour washing your own damn car - you might find the experience therapeutic.