--- title: What Are Shopify Checkout Functions? An Explanation for Merchants description: Shopify Checkout Functions enable deep customization of payments, shipping, and cart validation for Plus merchants. Learn what they are and how they work in canonical: https://tacey.app/blog/what-are-shopify-checkout-functions-an-explanation-for-merchants published: 2026-09-15T18:20:56.565Z updated: 2026-09-20T08:54:39.550Z category: News & Updates --- # What Are Shopify Checkout Functions? An Explanation for Merchants Shopify Functions allow developers to build apps that customize the checkout process, a feature once locked down, giving Plus merchants powerful new control. For years, the Shopify checkout was a fortress. Merchants could style it with their logo and brand colors, but the underlying logic was untouchable. This was a deliberate trade-off by [Shopify](https://www.shopify.com). In exchange for control, merchants received a checkout process optimized for conversion, security, and reliability. A benchmark study found its checkout outperforms competitors by [up to 36%](https://www.shopify.com/blog/what-is-benchmarking), a result of its relentlessly tested design. The risk of letting stores inject custom code into this critical path was too high. A single poorly written script could slow the page, create security risks, or break payments. That fortress wall, however, has been replaced by secure, controlled gateways known as Shopify Checkout Functions for Shopify Plus merchants. ## Why did Shopify change its checkout? Shopify evolved its checkout from a locked, one-size-fits-all system to an extensible platform using Functions. This change allows sophisticated merchants on Shopify Plus to meet complex business needs for shipping, payments, and promotions without sacrificing the checkout's core performance and security that the old system protected. The original "locked box" approach protected the checkout's integrity and performance for millions of merchants. Every millisecond of delay can lower conversion rates, and a security flaw could be catastrophic. By controlling the environment, Shopify guaranteed a fast, consistent experience, making its checkout a benchmark in ecommerce. However, as merchants grew more sophisticated, their needs surpassed a standard checkout. A brand selling heavy furniture has different shipping logic than one selling digital downloads. A B2B store needs to validate purchase orders. These complex rules did not fit the rigid structure, forcing merchants into awkward workarounds. Recognizing this, Shopify introduced Shopify Scripts for Plus merchants. This feature allowed them to write Ruby code to customize shipping, payments, and line items. Scripts were powerful but had significant limits. They were difficult to manage, test, and deploy, and they could break when Shopify updated the checkout. This led to Shopify Functions, a complete architectural rethinking. Functions are isolated pieces of code that run on Shopify's infrastructure, separate from the checkout's core logic. This separation is key to their power and safety. It allows for deep customization without compromising the speed and security of Shopify's checkout. ## What are Shopify Checkout Functions? Shopify Functions are small pieces of code, written by developers, that run on Shopify's backend to customize the checkout. They replace the old, brittle Shopify Scripts, offering a more secure, stable, and high-performance way to add custom business logic for payments, shipping, and cart validation. A Function executes a specific piece of business logic at a certain point in the checkout. For example, a Function could see an item in a cart and rename "Standard Shipping" to "Freight Delivery." The Function receives data from Shopify, performs its logic, and outputs proposed changes, which Shopify's systems then apply in milliseconds. Unlike old Scripts written in Ruby, Functions use any language that compiles to WebAssembly (Wasm), with Rust being the most common choice. This shift to Wasm is critical. It allows code to run in a sandboxed, high-performance environment, ensuring one merchant's custom logic never interferes with the core platform. The table below highlights the key differences between the old and new systems. | Feature | Shopify Scripts (Deprecated) | Shopify Functions | | --- | --- | --- | | **Language** | Specialized Ruby | Rust, or any language compiling to Wasm | | **Environment** | Ran on Shopify servers, less isolated | Sandboxed, high-performance Wasm environment | | **Deployment** | Copy and paste into Scripts editor | Deployed via a custom or public app | | **Stability** | Brittle, could break with platform updates | Forward-compatible with strongly-typed APIs | | **Failure Mode** | Silent failure where logic just disappears | Predictable, less likely to break silently | The danger of the old Shopify Scripts was their silent failure mode. A broken or deactivated Script often resulted in business logic vanishing without an error. For example, a "Buy One, Get One Free" promotion run by a Script could disappear. The checkout would still work but would charge customers for both items, leading to confused customers and brand damage. Functions directly replace the capabilities of the now-deprecated Scripts, offering several advantages: - **Performance:** Functions are compiled to a highly optimized binary format (Wasm) and run on Shopify’s global infrastructure, so they execute quickly and scale automatically. - **Developer Experience:** Developers can write, test, and deploy Functions using modern tools and workflows through a custom app, a more stable process than the old Scripts editor. - **Stability:** Functions are built against strongly-typed APIs, making them far less likely to break when Shopify updates its platform. They are designed to be forward-compatible. ## How do merchants use Checkout Functions? Merchants use Checkout Functions to solve specific business problems by embedding custom logic into the checkout flow. Common uses include customizing delivery options for certain products, reordering payment methods to lower fees, or enforcing minimum order values for B2B customers, creating a more tailored and efficient transaction. One of the most common applications is in delivery customization. For a store selling small accessories and large furniture, a Function can inspect the cart. If it contains an "oversized" item, the Function can hide standard mail options and only show "Freight Delivery." This prevents customers from selecting an invalid shipping method. This automation provides a clear return on investment. It eliminates the labor cost of a support agent manually correcting the order. It also avoids carrier fees for rerouting packages and reduces customer friction from a delayed shipment. The one-time development cost is often recouped by eliminating this persistent operational drain. Other powerful use cases for Functions include: - **Payment Customizations:** A merchant can reorder payment methods to feature one with lower transaction fees. They can also conditionally hide options, like hiding buy-now-pay-later services when a cart contains a gift card, to ensure compliance. - **Cart and Checkout Validation:** A B2B merchant can use a Function to enforce a minimum order value for wholesale customers. If the cart total is too low, the Function can prevent checkout and display a message explaining the requirement. > The checkout is where the promise of your brand becomes a real transaction. Giving merchants the tools to shape that moment to their specific business needs, without compromising speed or security, is a fundamental shift in how commerce on the platform works. Odera Joseph Echendu, Founder, Tacey These Functions can be combined for sophisticated experiences. A merchant could use one Function to validate that only tagged "members" can buy a specific product. Simultaneously, another Function could offer those members a unique "Free Member Shipping" option. This layering of rules allows merchants to tailor the checkout journey to an unprecedented degree. ## Do I need a developer to use Checkout Functions? Yes, implementing Checkout Functions requires a developer. They are not a setting you can configure yourself. Merchants can either hire a developer for a custom solution or use a third-party app from the App Store that provides a user-friendly interface to manage pre-built Functions. The power of Functions comes from their flexibility as actual code. This allows for nearly limitless logic. However, it also requires technical expertise in languages like Rust and an understanding of development workflows, command-line tools, and app deployment through the Shopify Partner Dashboard. This reality has created two main paths for merchants. The first is to hire a Shopify developer or agency to build a bespoke Function as a private app. This offers maximum flexibility but is the most expensive option. The second path is to use a public app from the [Shopify App Store](https://apps.shopify.com/) that uses Functions under the hood. These apps provide a simple interface to configure common rules, abstracting away the technical complexity. The trade-off is what can be called the "Abstraction Tax": you gain simplicity but surrender direct control. This becomes an issue when the app's logic fails in an edge case, as you are dependent on the third-party developer for a fix. For instance, a rules app might fail on a very large order because its Function is not optimized and exceeds Shopify's execution limit of [11 million instructions](https://shopify.dev/docs/api/functions/latest). The merchant is now at risk of losing a high-value order and must wait for the app's support team. This highlights the hidden risk of relying on a black-box solution for critical checkout logic. ## What problems do Checkout Functions not solve? Checkout Functions operate only before the sale is complete and cannot solve post-purchase issues. They cannot help a customer who needs to change a shipping address, edit an item variant, or add to an order after they have paid. These problems require different, post-purchase self-service tools. After an order is confirmed, a new set of issues can arise. A customer might notice they entered the wrong apartment number or chose the wrong size. Traditionally, resolving these issues requires contacting support, a manual process for the merchant and a frustrating delay for the customer. The next frontier of customization is enabling customers to manage their own orders after the sale. This involves making the order status page interactive. While a Checkout Function can validate a cart, it cannot help a customer who realizes they made a mistake in their shipping address right after placing the order. A post-purchase [order editing](/order-editing/) tool can allow that customer to correct their own address on the order status page. This self-service must happen within an "edit window" defined by the merchant, before the order is sent to fulfillment. This improves the customer experience and prevents costly mis-shipments. To enable this, a merchant must define their edit window by auditing their fulfillment latency. This is the time between an order being paid and a picklist being generated in the warehouse. A safe window balances customer convenience with operational integrity, preventing edits after an item has been picked. These post-purchase capabilities are outside the scope of Shopify Checkout Functions. They require a different set of tools that focus on modifying a confirmed order before it enters the fulfillment process. Combining Checkout Functions with post-purchase [features](/features/) creates a connected, end-to-end experience that reduces overhead and increases customer lifetime value. Instead of viewing your checkout as the finish line, consider it the starting point of the post-purchase relationship. Audit your support tickets. Solving the most common reason customers contact you after an order will often deliver a greater return than any optimization you can make before the sale. ## Frequently asked questions ### What did Shopify Checkout Functions replace? They replaced Shopify Scripts, an older, deprecated feature for Shopify Plus merchants. Scripts were less stable and could fail silently, whereas Functions provide a more secure, modern, and reliable way to customize checkout logic. ### Are Checkout Functions available on all Shopify plans? No, Checkout Functions are a feature available exclusively to merchants on the Shopify Plus plan. This tool is designed for high-volume and complex businesses that require a greater degree of customization in their checkout process. ### What programming languages are used for Functions? Functions can be written in any language that compiles to WebAssembly (Wasm). Rust is the most common and best-supported language, chosen for its performance and safety, which helps ensure custom code does not compromise the checkout experience. ### Can I set up Checkout Functions myself without coding? No, you cannot configure Functions directly in the Shopify admin as they require custom code. To use them, you must either hire a developer to build a private app or install a public app from the Shopify App Store that manages the Functions for you. ### What is an example of a delivery customization with a Function? A common example is a store that sells both small items and large furniture. A Function can check the cart and, if it contains an oversized item, automatically hide standard shipping options and only show a "Freight Delivery" option to the customer. ### Can Functions help if a customer enters the wrong shipping address? No, the scope of Checkout Functions ends once the payment is complete. To solve post-purchase issues like a wrong address, you need different tools that enable customer self-service on the order status page, allowing them to make edits before the order ships. ### What is an "edit window" for post-purchase changes? An edit window is a specific period after an order is placed during which a customer can make changes themselves. This window's duration is based on the merchant's fulfillment latency, ending before the order is sent to the warehouse to ensure operational integrity.