"Every 6th wash free" — a punch card the customer will lose. And you don't even know how many regulars you have.
Instead of punch cards — Wallet. Instead of stamps — 5-10% cashback. You know every customer: when they came, how much they spent.
Push after 2 weeks: "Time for a wash? You have €4 in bonuses." Seasonal promotions for members. Upsell premium wash packages.
Average customer — 2-3 times per month, €10-20 per visit. €350-700 per year. Without bonuses, they'll switch to the competitor by month three.