This happens more often than you would expect, and if it has not happened to you yet, it probably will. A client, or someone walking by while you are working, notices the fragrance and asks, directly, if they can just buy the one hanging in the car.
It is a strange position to be in, mid-job, with someone trying to purchase a small accessory on the spot. It is also one of the clearest signals you will get that a product is working exactly as intended, strong enough to be noticed by a total stranger walking past, not just the client who booked the service.
The practical fix is simple: keep a small stock of extras on hand specifically for this. Selling one off your kit at a modest markup covers the moment without disrupting the job, and it turns a random passerby into a lead for your actual detailing services, not just a scent sale.
There is a bigger opportunity underneath the immediate transaction too. That moment is proof, in real time, of exactly the kind of word of mouth this fragrance is built to generate. Someone who bought their own hanging Drive scent off a detailer they were not even a client of is a customer you acquired without spending a cent on marketing.
If this has happened to you, it is worth treating as more than a funny story. It is a live demonstration of the referral math working exactly the way it is supposed to.