UK Shopify app development means building the custom logic that no existing Shopify app handles — VAT reclaim rules, Royal Mail API, and CSS partner checkout routing

A UK VAT exemption flow for B2B buyers requires three components: a checkout UI extension that collects the buyer's UK VAT registration number, a backend validation step that queries HMRC's VAT validation API (https://api.service.hmrc.gov.uk/organisations/vat/check-vat-number/lookup/{vrn}) to confirm the number is valid and active, and a Shopify Cart Transform Function that removes the VAT line item from the cart when a validated VAT number is present. The VAT number and validation status are stored as cart attributes. This is a Shopify Functions-based implementation — the deprecated Shopify Scripts approach does not support this correctly.

Royal Mail and Evri calculated rates are integrated using Shopify's Carrier Service API. The integration is a private app that: registers a carrier service with Shopify via the Admin API (POST to /admin/api/2024-01/carrier_services.json), provides a publicly accessible HTTPS endpoint that receives Shopify's rate calculation request (containing the order's weight, dimensions, and destination postcode), queries the Royal Mail Click and Drop API or Evri API for live rates, and returns the rates in the Shopify carrier service response format. The endpoint must respond within 10 seconds or Shopify times out the rate request. Shopify Plus is not required for carrier service API access; it is available on the Advanced plan.

A Shopify Function is custom logic that runs on Shopify's infrastructure at checkout — for discounts, shipping, payment customisation, cart transforms, and order routing. You need a Shopify Function rather than a discount app when the discount logic involves conditions that app-based discount rules cannot evaluate: customer tag combined with product tag combined with currency, volume tiers with product category exclusions, or B2B pricing that must be applied silently without a visible discount code. Shopify Functions are deployed via the Shopify CLI and associated with a Shopify app (public or private). They execute at checkout with no external API calls and no latency added to the checkout flow.

Connecting Sage 200 or SAP Business One to Shopify for inventory sync requires a custom integration app that uses Shopify's Admin API (specifically the Inventory Level API and Product Variant API) and the ERP's API or database connection. The integration handles three data flows: inbound inventory updates from the ERP to Shopify (triggered by ERP stock movements via webhook or polling), outbound order creation in the ERP from new Shopify orders (triggered by Shopify's orders/create webhook), and product data sync (SKU, price, and description updates from the ERP to Shopify). The data mapping — particularly for SKU alignment, location mapping, and multi-currency pricing — is typically the most complex part of the build.

A Shopify public app is listed in the Shopify App Store (or distributed via a direct install link) and can be installed by multiple merchants. It uses OAuth for authentication and is built to handle multi-tenant data. A private app (now called a custom app in Shopify's current terminology) is built for a single merchant store, authenticated with an API access token generated in the store's admin, and not listed in the App Store. For UK brands needing a custom ERP connector, a bespoke VAT integration, or a proprietary carrier rate tool, a custom (private) app is the correct approach — it does not require App Store review and can access all Admin API scopes the merchant approves.