Billing API: How to make Recurring app Charge ONLY to NEW customers and Free to pre-existing ones?

Hello everyone,

I am writing to know how to proceed with a rejection when attempting to make our SimpliRoute application public in the Shopify store.

SimpliRoute is a Last-mile logistic solution that allows our customers to manage their dispatches and create optimized routes for them, so we decided to create a Shopify plugin/app to those customers who have a Shopify store so they can send their orders directly to our platform trough our API.

We wanted to make this intermediate app/plugin for free, but we have rejected to make it public because we’ve been asked to use the Shopify Billing-API to make payments to our service. However, our intention is that customers who are already using our service (and therefore paying for it in a separate billing system) and have a Shopify store can also use this intermediate Shopify app developed by us for free.

I sent this restlessness to Shopify’s contact support and I’ve got this answer:
“…We do understand that many partners, such as yourself, may have pre-existing customers who are on a separate billing system. We are not requiring these customers to be billed through the Shopify API. The only requirement would be that new customers signing up through Shopify would be billed through the Shopify billing API. So you would be looking to build the app in a way to exempt pre-existing customers from payments…” “…taking a look at our developer forums might be a good way of finding out how other developers in this situation have proceeded with their apps.”

That being said, I haven’t found something like that in the forum, and we need a solution about it. Please someone could help me with this case?
Thank you very much in advance.