Budget: 500 UAH Deadline: 1 day
It looks like an idea for a startup. So most likely it will start with an MVP.
The first thing to determine is what minimal set of functions the application should perform and make calculations based on that.
No one will provide accurate calculations without prototypes and descriptions, if they do, it's either sellers of ready-made solutions, or people who don't know what they are talking about.
The advantages of ready-made solutions are - cheap and quick testing of the business model, understanding whether "it will work" or not. The disadvantages - if "it's worth it" everything will have to be rewritten from scratch in the chosen technologies.
Online payments with subscriptions provide many LPAT systems, some have auto-signing on a schedule (e.g. WP), in others (e.g. Platon) there is the possibility of debiting but the debiting is initiated on the application side and not on the payment side, which requires additional programming, and therefore increased costs.
Mailings - it is most convenient to connect an external service like Sandgrid and use its API capabilities. If the service exceeds the limits, the service provides free shipments.
This is only part of the answers to your questions, I can also answer other questions in more detail. I recommend choosing several freelancers for consultation and talking to each one separately. This is much cheaper than any mistake in choosing the development path or during development.
