Implement Shopify "flex billing" in an app — flexible tiered app subscriptions with in-place upgrades/downgrades, proration, and automatic usage-based tiering, all without triggering Shopify's subscription re-approval flow. Use when the user wants to add flexible/tiered Shopify billing, charge usage records against a capped usage line, pro-rate plan changes, auto-upgrade on usage limits, or migrate off Mantle Flex Billing into their own codebase.
