Hi there,
Thanks for your suggestion.
There are two options that WHMCS facilitates:
1. With your cost being incurred on a monthly basis on a potentially variable basis, using the same billing cycle for customers would be the best option to avoid this potential for disparity.
You can create a separate product for a cPanel License product with just a monthly billing cycle (select the AutoRelease module) and assign the cPanel license tier configurable options to it.
In this way the customer can purchase the cPanel License product on a monthly basis, and continue being billed long longer cycles for the main product.
2. Switch off the Auto-Scale option, so that you and customers have a predictable regular price. Clients can place upgrade/downgrade order for the license tier via the client area, and this will be propagated to the cPanel license.