Isn't this pretty much what every processor everywhere charges? I have been out of the industry for 10+ years but I remember this being pretty standard.
Isn't Stripe Billing only for subscriptions?
To only pay Stripe 2.9% + $.30 they'd need to only use Stripe just for charges, and either build their own billing engine (usually poor use of time) or use a 3rd party SaaS.
As we approach Black Friday in the US, I'm thinking of the giant volume of e-commerce transactions that are not subscriptions (or usage-based), and of which Stripe gets a cut. It sounds like you are not angling for that part of the market?