Fix Shopify App Billing Integration

I’ve already built a connector app that lets merchants link their Shopify stores to my SaaS platform—data sync works perfectly. The road-block is the Billing API: every attempt to create a one-time charge runs into setup errors, charge approval failures, and odd glitches with what should be a simple “recurring billing disabled” flow.

You’ve shipped Shopify apps before and know the Billing API inside out, so here’s what I need from you:

• Diagnose why the initial charge creation is failing and trace any mis-configurations in the app setup or callback URLs.
• Implement a clean, compliant one-time charge flow that passes Shopify’s approval, surfaces the confirmation screen correctly to the merchant, and records the charge status on my side.
• Resolve the recurring-billing-style webhooks that Shopify still seems to fire, ensuring merchants are never double-charged.
• Provide a quick hand-off note or loom explaining where the fix lives in the codebase so my team can maintain it.

I’ll give you repo access, existing Billing API code, and a test store. Once the first successful charge goes through on that store and we can replicate it, the job is done.