Please consider integrating any payment system, especially STRIPE. They have a flexible API and webhook system for integration. Metabox already has the MB custom table, user profile and front-end form submission feature.
STRIPE also has an API for the customer portal, so you do not have to create a complex dashboard or billing system, or even manage email deliveries.
Metabox users are definitely happier and you can grow your customer base from 800,000 to more.
In any case, congratulations to Mr. Tran and his team for this success.
It is an interesting idea to create an integration between MB Frontend Submission with Stripe to allow site owners to charge users per post submitted. I will inform the development team to consider adding this feature request to the development queue.