A car wash for my truck costs me anywhere from $15-$19 for a "hand wash" at most of the car washes around my home and office; the car wash in the bottom of my building, which is a true hand wash (they use buckets of soap and water, no mechanical devices) costs about $15. At almost all of the car washes I go to they use compressed air to blow water from around door frames and out of cracks: areas that they could not reach with towels or cloths. Because they are already using compressed air, how hard would it be for them to correctly adjust the air pressure in my tires? I think I would pay an extra $1 for a quick air pressure refill. Lots of people I know would pay a small extra fee for an air pressure fill because I know that the only time their air pressure gets checked and adjusted is when they get an oil change; believe me, your air pressure needs to be adjusted more frequently than every 3,000 miles.
Even better than charging an extra $1 for the service, just make it a free prize with the expensive package. On the menu of options, just add another bullet point that says "Tire pressure checked." Once people figure out that there's a tire pressure adjustment, they might just upgrade from the $9.95 basic wash to the $18.95 premium wash. The great part is that if the car wash already has compressed air at the drying stations, it's a very small investment in a tire filler adapter and a gauge for each station -- maybe about $15 per station.