How to use Billing API for FREE SMS Application and Shopify Commission Percentage charges

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Need clarification and understanding on the use of Billing API.

We are developing an application for our client who wants to place it as a FREE application on the Shopify marketplace.

The client is into SMS packages selling in the MEA region.

Customers buying SMS packages are sending SMS to their Shopify Store registered customers.

The client wants its customers to use the Shopify application to send SMS directly using the Shopify store, if any end-customer registers, place the order, or cancels the order.

Also, clients customers can send promotional SMS using Shopify by exporting customers list from Shopify Admin.

Shopify Application would be downloaded for FREE from the marketplace and customers can set up an account using credentials provided to them.

After they log in, available SMS credits can be seen in the Shopify admin.

They can set up SMS templates in the backend and use them to send SMS.

The client has APIs available which are used to trigger SMS on every event happening in the Shopify store.

If the customer has to buy more credits, they would buy credits from the client outside the Shopify store as they need to sign the agreement as per telecom regulatory directives.

Also, the client is not having a payment API for SMS credit purchases as the online purchases can result in SPAM messaging if unknown users buy it online.

We learned that Billing API is a requirement to get the app approval.

In the above case,

How do we use the Billing API?

How commission percentage is charged by Shopify if the app is FREE to download?  What is the commission percentage?