Shopify Custom Checkout App

I need a Shopify custom app that fully replaces the standard checkout so I can control three key areas: the payment options shown to shoppers, the look-and-feel of every checkout screen, and the shipping methods that appear based on dynamic rules.

Beyond Shopify’s own APIs, the app must hook into a few external services (payment gateway, rate-shopping API, and our CRM for post-purchase tagging). Clean, well-documented code and a simple settings panel in the Shopify Admin are essential so my non-technical staff can toggle features without editing code.

Deliverables
• Private or public Shopify app (Node or Ruby) installed in my test store
• Custom checkout flow implementing my UI mock-ups, alternative payment gateway, and conditional shipping rates
• Secure integrations with the specified third-party APIs (keys provided)
• Written install / uninstall instructions and brief video walkthrough
• One round of post-launch tweaks based on real-store feedback

Acceptance Criteria
• Checkout passes Shopify compliance checks and processes real orders end-to-end
• Payment, UI, and shipping customisations match the approved design spec
• All third-party calls succeed in staging and production environments with <300 ms average latency
• No console errors and Lighthouse performance score above 85 on checkout pages

Timeline is flexible but the core flow should be demo-ready within three weeks. I’ll be available daily for quick answers so development can move fast.