Design a reservation and payment system for a parking garage.

Focus on the user experience for both customers and garage operators. Start by outlining the key user journeys, such as making a reservation, checking in, and processing payment. Consider the technical requirements for real-time availability updates and payment processing, as well as how to handle cancellations or changes to reservations. Discuss the integration of mobile and web interfaces, and think about security measures for payment information. Finally, address potential scalability issues if the system needs to accommodate multiple garages or varying demand levels.