UPDATE: I've solved this issue, in case anyone stumbles across this in the future. I set my redirectUrl in my initial appSubscriptionCreate mutation to be `https://<myapphost>.ngrok.io/?shop=${shop}&host=${host}`, and that seems to have fixed my issu...

So I've created a Shopify app using the Ruby shopify-cli gem, and I am working on implementing the Billing API now. I'm using the scaffolding code created by the shopify node create command, so it's a generic GraphQL API mutation using the pre-built ...