You have the flexibility to set your pricing structure and you can continue to bill your customers directly/offline. However, it's advisable to consider industry standards. Many successful agencies opt for one-time payments to provide their customers with convenience.
Here are some advantages of charging a one-time payment:
Instant revenue: You receive the full payment upfront, ensuring immediate revenue for your services.
Simplicity: Billing is straightforward for both you and your customers, eliminating recurring billing concerns.
Lower churn: Typically, lower customer churn as they've invested in the app.
Charging in alignment with industry norms can enhance the perceived value of your services and simplify the billing process for both you and your customers. However, it is prudent to consider that an average consumer spent approximately $50,000 to develop a mobile app in 2022.
Ultimately, the choice of how to charge your customers should be based on your specific business model, goals, and your understanding of your customers' preferences.