You can offer selected current subscribers a reduced subscription rate. Vendors typically do this to reward loyal users, to keep users during a system drawdown, or to "grandfather" in current subscription prices for current users when increasing the price for new users.
To set up custom billing,
1) Go to your system page and click the SUBSCRIBERS button.
2) This will take you to the Subscriber Management page.
3) Click the Create Custom Billing link for your selected subscriber and enter your custom price and number of billing cycles.
Please note that any custom billing you set up will override any price increases you subsequently impose on the system in general, and the custom pricing will last until you specifically remove it or until it finishes the number of billing cycles.