Shopify Functions Explained: Custom Discounts and Checkout Logic Without Apps
Shopify Functions let you customise discounts, shipping, payments and checkout validation with code that runs on Shopify's own infrastructure. Here's what they do and when to use them.
If you have ever wanted your Shopify store to do something the settings don't allow, like a "buy 3, cheapest free" discount or hiding a shipping method for certain carts, the answer in 2026 is Shopify Functions. They replaced the old Shopify Scripts and are how custom backend logic works now.
What Shopify Functions are
Shopify Functions are small pieces of custom logic that run on Shopify's own servers, deep inside the checkout and cart. Instead of a bulky app bolting behavior on from outside, a Function changes how Shopify itself behaves, quickly and reliably.
They replaced Shopify Scripts, which were deprecated along with the old checkout. Functions are faster, more powerful, and work with the modern Checkout Extensibility system.
What you can customise with them
Functions cover the areas merchants most often want to bend to their business:
- Discounts: complex promotions beyond the built-in rules (tiered, bundle, "spend X get Y", customer-specific).
- Shipping / delivery: rename, reorder, or hide delivery options based on the cart, address or customer.
- Payments: rename, reorder or hide payment methods (for example, hide cash on delivery above a certain value).
- Cart and checkout validation: block checkout when rules aren't met (minimum quantity, restricted product combinations).
- Cart transforms: bundle products or adjust how items appear in the cart.
Why this matters for your store
- No heavy third-party app for logic Shopify can now handle natively, which means fewer scripts slowing your store and fewer monthly fees.
- Runs at Shopify's speed, inside checkout, so it's fast and reliable even at high traffic.
- Upgrade-safe, because it fits the current Checkout Extensibility model rather than hacking the old checkout.
When you need a developer
Functions are written in code and deployed through a small app using Shopify's CLI. They aren't a settings toggle. That's the trade-off: far more power than the built-in rules, but you need a developer to build and deploy them.
The good news is that once a Function is live, it just works in the background, and you manage its behaviour from the admin.
Should you use them?
Reach for a Function when:
- The built-in discount, shipping or payment settings can't do what you need, and
- You'd otherwise install an app (or several) just for that one behaviour.
For simple stores, the native settings are often enough. But as soon as your promotions or checkout rules get specific, Functions are the clean, fast, native way to do it.
The takeaway
Shopify Functions are how you customise the parts of Shopify that used to require clunky apps or the retired Scripts. Custom discounts, smarter shipping and payment rules, and checkout validation, all running natively and fast.
Want a custom discount or checkout rule your current setup can't handle? Tell me what you need and I'll build it with Shopify Functions.