Hello!
I am the co-founder of GoodAPI. We help brands streamline their climate and social impact programs. We use Billing API usage based pricing to charge for trees and plastics and have to rely on Stripe to collect donations. Terrible UX 
My BIGGEST feature request for the new Shopify Billing is to allow us to tag certain usage charges as “donations” so that they can get treated separately i.e. They don’t get charged under the 15% partner revenue share and can be passed through directly to our non-profit partners. That way we won’t need to create this hodge podge of Billing API for subscriptions and Stripe Billing for donations. Pretty please!
Mock Example
Instead of sending a $0.50 usage fee under Billing API, i would love to send
$0.48 donation, $0.02 fee
And treat the $0.02 as our revenue that should be subject to the partner revenue sharing and any other shopify fees. While the $0.48 is passed through directly so we can send it to our non-profit partners.
Happy to work with the team on this. We will migrate Yesterday if this was possible!