(PM from Stripe) If you're an existing user, there's no extra fees, even if you are past the $1M threshold. For existing Stripe Subscriptions users, Stripe Billing is included in your current price. (You should have received an email about this earlier today)
The initial communication left some room for clarity, which really had me confused in the same way as others in this thread. Really outside of the norm for Stripe, who's communication is usually very clear.