You can self-serve manage your Betafi paid plan billing details from the Billing Settings Page:
Plans are billed either Monthly or Annually based on your preference on signup.
You can manage, update your active plan, and more via the Change Plan or Billing Details button once you have an active paid plan:
If you have received a partner promotion code eg via PitchGround, you can redeem it via the Redeem Partner Coupon form: