Billing API appUsage

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Hello everyone, I started working on an app couple of weeks ago that is mainly responsible for donations by offering a specific product while the customer is checking out.
Till now I was able to make this offer to ask the customer about the donation and successfully add it in the cart upon approval but I am stuck to make app usage subscription and to link my product by the amount the store should pay me monthly.
following Shopify dev docs I made app usage details posting to the API but I can't link it with the product and test it. Any help with that? I followed the NodeJs/React tutorial to create this app.