If the pricing option "Yes - Scaling Service: Each service instance allows a quantity to be defined" there should be a way customers can edit this qty in the client area that would also trigger the relevant billing.
When a client signs up for the service they specify a quantity. i.e. 5. There should then be a way for the client to adjust the quantity to 6, 10 or any other amount. We can adjust this qty manually in the admin panel, but that then whmcs does not do the relevant billing for the adjustment until the next month (Assuming we do the auto recalculate on save).
In our specific scenario:
1) we sell Microsoft 365 licensing. A customer orders Microsoft 365 Business Standard and specifies a quantity of 5 because they want 5 licenses when they place the order. This is perfect.
2) the client has a new staff member and wants to increase the quantity to 6. This is where the issue comes in as there is now way to change quantity other than manually on the admin backend.