I am developing a SaaS application that will be paid from its first public release, and I am planning a free trial period as part of the subscription model.

I want to ask about the recommended timing for integrating a payment gateway (such as Stripe or PayPal) relative to developing the core features of the application.

Specifically:

At what point in the development process should a payment gateway typically be integrated in a SaaS project?

Are there established practices or guidelines for whether a payment integration module is developed early (in parallel with other features) or later (after core functionality is complete)?

What factors do developers or teams consider when deciding the timing of payment gateway implementation?

Thanks to everyone!