Nick here from Shopify. Great question!
I looked into this for you and with this particular app, it is a billing related app which has % fee's on top of each order made as it's a subscription app. Because of the % fee, it needs to be billed externally from Shopify.
This is also explained a little more below the app description which you can also see below:
I hope this helps and answers your question.
All the best, Nick
Good question. I'm not completely familiar with how API and billing with apps work exactly. Shopify does have a Billing API help document which you might find useful though. You can find it here.
Hopefully, this helps answer some of your questions.
If for some reason it’s a charge that can’t be on the billing API (for example the products' the merchant's order through a drop shipping app, or when merchants purchase ads through an app) then Shopify initiates a rev share agreement. Something to remember is it can't be offered for free and hoard the money. Your best bet to ask someone about your eligibility for this is through partner support located in your partner dashboard. Those tickets would directly to the correct team you are looking to talk to about this.