How can I add a line item at subscription creation?

Hi,

In my application releasenotes.io, my billing is such that there's a base plan, then an additional charge depending on how many subscribers you have on your account, in batches of 1000.

I'm able to us the Stripe webhook for account renewals, to add a line item to the invoice before it's finalised, however I'm struggling to find a way to add the line item to the invoice when a subscription is created or switched to a new plan, since in both those cases it's done immediately at Stripe so there's no time to update the invoice.

Ideally, I'd like to just override the billing controller so I can do what I need from the controller as the subscription is being setup / switched.

Any ideas on how I can achieve this?