Optional service multiplying by tire quantity instead of charging once per vehic

Hello,

We have a tire shop configured with HikaShop where customers can purchase tires and additional services. Most services are charged per tire unit (mounting, nitrogen inflation, etc.), but we have one specific service that should be charged once per vehicle, regardless of tire quantity: "Steering alignment/geometry for passenger cars".

Current problem: This service is configured as a product characteristic, and when a customer adds 2 tires to cart and selects this optional service, the system multiplies it by 2 (85.95 € × 2 = 171.90 €), when it should only charge 85.95 € once, as this service is performed on the vehicle as a whole, not per individual tire.

What we need:

This service should remain optional (not mandatory)
It should be visible always, even when buying a single tire
When selected, it should add only 1 unit to cart, no matter how many tires are being purchased
All other services should continue working per tire unit as they do now

Question: How should we configure this service to charge it once per vehicle instead of multiplying by tire quantity? Should we use a different configuration method (option instead of characteristic, or separate product)?

Please see attached screenshots:

Product frontend view showing the service
Cart showing the service multiplied by 2
Backend product configuration

Thank you for your assistance.