Add-on [PAID] API to add, remove or update User Upgrades

We would like to handle payment on our own site and then use an API installed at our Xenforo site to add, remove or update the User Upgrade.

Using Contex's Xenforo API we can easily add a secondary usergroup when someone purchases an upgrade from our site. However, we would like to also add a User Upgrade in Xenforo so that when the time expires, the user is downgraded and the secondary usergroup is removed.

The requested User Upgrade API should have the same functionality as the Manually Upgrade User within adminCP.

We offer recurring billing, so the API should be able to update an existing User Upgrade to extend the end date.

Finally, the API should allow to Downgrade and active User Upgrade in the same way as can be done in adminCP.

Please get in touch with an estimate of cost, and let me know if you have any questions.

Thank you for reading. :)