Grocery shoppers do not want a beautiful checkout. They want to know whether the coriander will arrive tomorrow. If the answer is no, they want to know now — not after they have added fourteen SKUs.
On Organic Tattva we placed serviceability checks at product and cart, not only at checkout. The input is a pincode. The source of truth is the coverage the logistics partner and OMS actually honour, not a static list someone updated last monsoon. The storefront asks, the custom app answers, and the buy path either proceeds or explains the gap.
Implementation notes that matter: cache positive and negative answers with a short TTL so a collection page full of add-to-cart buttons does not hammer the API; show a clear unserviceable state instead of disabling the button with no copy; keep the check out of the critical rendering path so LCP does not wait on logistics.
Then the order has to land in the OMS without a human. Serviceability that is honest on the site and ignored in operations is how you get refunds. The Shopify custom app, the logistics integration and the OMS sync are one design, not three tickets.
If you sell perishables, bulky goods, or anything with a delivery radius, this is not a ‘nice widget’. It is the difference between a store that takes orders you can fulfil and a store that trains customers to distrust the promise.