Enhanced Shopify App for Country-Specific Shipping

I already run a custom-built Shopify app that connects multiple suppliers to my store. I now need a focused update so shipping can be limited to suppliers and customers who share the same country. The core change is to let shoppers actively choose which countries their order may ship to, right in the checkout flow. If someone selects a destination that isn’t on the permissible list, an immediate error must appear at checkout and prevent completion of the order. Beyond that key feature, I have a short list of minor tweaks and previously noted fixes that we can review together once you’re onboard, but nothing as large as the shipping rule. Deliverables • Checkout-level UI element where the customer selects allowable shipping countries • Server-side validation that blocks orders outside the chosen list and returns a clear error message at checkout • Admin setting so I can edit / add countries per supplier without deploying new code • Full test coverage to prove the rule works with multiple suppliers, draft orders, and accelerated payment methods Please keep everything native to Shopify’s current API versions (Functions, Checkout UI extensions, etc.) so future upgrades stay painless. Once the change is live, I’ll need a brief walkthrough of the new controls to hand to my support team.