# Custom Shopify App Development

> Custom Shopify app development: custom or public app, Shopify Functions, checkout and theme extensions, GraphQL integrations. Fixed quote, source code handed over.

- Page: <https://www.datafirefly.com/en/expertise/shopify-app-developer/>
- Language: en
- Last updated: 2026-09-24
- Other languages: [fr](https://www.datafirefly.com/expertise/developpeur-app-shopify/index.md), [es](https://www.datafirefly.com/es/pericia/desarrollador-app-shopify/index.md), [de](https://www.datafirefly.com/de/fachkompetenz/shopify-app-entwickler/index.md), [it](https://www.datafirefly.com/it/competenze/sviluppatore-app-shopify/index.md), [pl](https://www.datafirefly.com/pl/ekspertyza/programista-aplikacji-shopify/index.md), [nl](https://www.datafirefly.com/nl/expertise/shopify-app-ontwikkelaar/index.md), [pt](https://www.datafirefly.com/pt/expertise/programador-app-shopify/index.md)
- Index: <https://www.datafirefly.com/en/expertise/llms.txt>

Shopify cannot be modified the way PrestaShop or WooCommerce can: no access to core code, no database to extend. Everything goes through apps, extensions and Functions, within a framework set by Shopify. It is a constraint, but also a guarantee: a well-built app does not break when Shopify updates the platform. We build custom Shopify apps inside that framework, with a written scope and a fixed price agreed before we start.

## Custom app or public app

| Format | Who it is for |
| --- | --- |
| Custom app (private) | Your store only, installed directly without going through the App Store. Ideal for automation, a business-specific admin screen or an integration with your internal tools. |
| Public app | Listed on the App Store for other merchants. It involves billing through the Billing API, an onboarding flow, a listing and Shopify's app review. |

## What we build on Shopify

### Shopify Functions

Business logic executed by Shopify at cart and checkout time: custom discounts, hiding or renaming delivery and payment methods, cart validation, line bundling. Functions replace the former Shopify Scripts and work on every compatible plan, not only Shopify Plus.

### Checkout extensions

Fields and blocks in the checkout, the thank-you page and the order status page, through Checkout UI Extensions. If your customisations still relied on checkout.liquid or additional scripts, we inventory them and rebuild them in this model.

### Theme app extensions

App blocks added to the theme without touching its code, placed by the merchant in the theme editor. The theme can be updated without losing the feature.

### Admin and integrations

Screens embedded in the Shopify admin, sync with an ERP, a PIM, a 3PL or an invoicing tool through the GraphQL Admin API, webhooks and bulk operations for large catalogues. Metafields and metaobjects structure the data specific to your business.

## How we write a Shopify app

- **Shopify's official template** (React Router, formerly Remix) and Shopify CLI, to follow platform changes without rewrites.
- **GraphQL Admin API** with an explicit API version and a plan to upgrade it every quarter.
- **Verified, idempotent webhooks**: the same event received twice never triggers two jobs.
- **GDPR compliance webhooks** implemented, mandatory for public apps and recommended for custom ones.
- **Polaris interface** in the admin, so the app looks like the rest of Shopify.

## What you receive at delivery

- The app deployed and installed on your store, with its Git repository.
- The complete source code with no restrictive licence.
- Operations documentation: hosting, environment variables, deployment.
- 12 months of compatibility updates, including Shopify API version upgrades.

## What drives the price of a Shopify app

A custom app starts at €4,000. A public App Store app usually lands between €15,000 and €50,000 depending on scope.

| Factor | Effect on the quote |
| --- | --- |
| Format | A custom app avoids billing, onboarding and App Store review. |
| Extension points | A single Function is quick; an app combining Functions, checkout extensions and admin screens takes longer. |
| Integrations | A documented API, or high-volume sync with bulk operations and retries. |
| Hosting | The app needs a server; we set it up or deploy on your infrastructure. |

## Shopify, PrestaShop or WooCommerce?

If your need requires changing the store core or storing complex data, an open-source platform may fit better. We compare the options on our [PrestaShop vs Shopify](https://www.datafirefly.com/en/expertise/prestashop-vs-shopify/) and [WooCommerce vs Shopify](https://www.datafirefly.com/en/expertise/woocommerce-vs-shopify/) pages, and also handle [Shopify to WooCommerce migration](https://www.datafirefly.com/en/expertise/shopify-to-woocommerce-migration/). For automation and AI on Shopify, see [Shopify automation and AI](https://www.datafirefly.com/en/expertise/shopify-automation-ai/).