Reputable shops I more often see do something more like a 'Prepaid' discount where you get X washes (maybe in the next Y months) for Z dollars, and ideally it's something like you get one wash a month at a 10-20 percent discount.

The profit-gouging ones, either do a 'assume 3-4 washes a month to see real benefit' or assume you are washing once a week in their sub... or do all the other 'tricks' above schemes can allow.

But I never see anyone using the subscription car wash in town, so who knows? The new ones connected to the gas station see some action.