Availability is the hard part, not the checkout
Every booking system can take a card. What separates them is how honestly they model availability: guides, vehicles, equipment, and the departure itself are separate constraints, and a tour is only bookable when all of them line up.
If your system tracks seats but not the fact that you own three vans, you will eventually sell four simultaneous departures. Resource-based capacity — where a shared asset reduces availability everywhere it is used — is the feature that prevents the phone call you never want to make.