You can use any Joomla Subcription component to
- receive payments and ... add the paid user into USER-GROUPs that are privileged via ACL to add / edit content
- About redirections to payment gateway,
the redirection to the payment getway and back are handled by Joomla subscription component
- about redirections handled by FLEXIcontent, you can configure people trying to add CONTENT items without privilege
a. if not logged then to register form
b. if logged then you can redirect to a custom page, e.g. to your subscription component