Verifying an address matches its stated postal code before checkout
A mismatched postal code and city on an order form looks like a small typo until it turns into a delivery sent to the wrong part of the country entirely.
Estimating shipping cost from postal code alone
Asking for a full address before showing a shipping estimate loses shoppers who just want to know the number before committing to a purchase. An online store selling bulky items, where shipping cost varies a lot by distance, wanted to show a reasonably accurate estimate from the smallest possible amount of information a shopper would be willing to type in early.
A postal code turned out to be that minimum. A single field, quick to type, and specific enough to locate a shopper within a meaningful area for shipping purposes, without asking for a street address a shopper might not want to commit to before deciding whether to buy anything at all. The store added a "check shipping cost" field right on the product page, well before checkout, that took only a postal code.
Behind that field, /v1/postcode took the entered code and returned what it corresponds to. The store used the resulting location to calculate distance from its own warehouse, feeding that distance into its existing shipping rate table the same way a full address would have, just with a coarser starting point. For a bulky-item retailer where shipping is priced primarily by distance band rather than exact address, a postal code was precise enough to place a shopper in the right band the overwhelming majority of the time.
The store was upfront that this was an estimate, not a locked-in quote, both because postal code areas can be large in less densely populated regions and because the final shipping charge still depended on the exact delivery address entered at checkout. Framing it as an estimate up front meant nobody was surprised when the checkout page showed a number that differed slightly once a full address replaced the postal code guess.
This approach paid off most clearly on the product page, where showing a shipping estimate before checkout measurably reduced the number of shoppers who added an item to their cart only to abandon it once they discovered the shipping cost at the very last step, a well-known source of cart abandonment. Moving that information earlier, even as an estimate, let shoppers self-select out earlier if the total cost was not going to work for them, rather than after they had invested time filling out an entire checkout form.
Because the postal code check ran on the product page rather than at checkout, it saw more traffic than a checkout-only integration would have, since browsing shoppers who never buy anything still triggered it. That pushed usage past the free daily allowance during busy shopping periods, and the store settled into prepaid credit at €0.0001 a request for the difference, a cost small enough against the value of reducing last-minute cart abandonment that it was not a close decision.
A postal code is a small piece of information, but it is often the largest amount of location detail a shopper is willing to give before they have decided to buy anything, and building an estimate around exactly that much data respects where a shopper actually is in their decision. Documentation for the endpoint is at /docs/postal-code-lookup/.