Stripe Subscription Adjustment based upon usage of second product

Question

Here is an example of what is going on and what I need to do.

Lawyer pays for legal services monthly website for $39. A second product of the website is to monitor legal cases (like a divorce so the lawyer knows what is filed). Each monitored case is $4.99 but the first 5 cases are free as part of the $39 subscription. The lawyer can use the $4.99 as a client expense with a receipt specific to that client. Creating a separate subscription specific to each client name is easy and does the proration when started in the middle of the billing cycle. But when I start a subscription for the first 5 cases, that's an additional $24.95 (4.99 x 5). The total should still be $39, not $39 plus $24.95.

I tried coupons, but that didn't prorate the base $39 for the time left. Is there a way to apply the coupon back to the beginning of the month?

My other thought was to adjust the price of the underlying subscription for $39.00 and make the new one $34.01, then take off another $4.99 when 2 cases are monitored and finally on the 6th case, that's beyond the amount included so the 6th case would be charged $4.99 and so on for any additional cases.

What would be the best way to structure this pricing model in Stripe?