What is a Shopify Checkout Extension?

Shopify Checkout UI Extensions are sandboxed app components that let you customize the appearance and functionality of the checkout without compromising customer data security. They run in an isolated environment, separate from the checkout page and other extensions, with no access to sensitive payment information or the checkout page HTML.

Think of it this way: your Shopify checkout has a set structure, customer info, shipping, payment, order summary, and Shopify controls that structure. You can't inject random code the way you used to with checkout.liquid (which is now deprecated). Instead, you add things in specific, defined slots using apps built on the UI Extension framework.

Three extensibility surfaces in 2026

In 2026, the term covers three extensibility surfaces that work together:

• Checkout UI Extensions : React-style components that render in checkout slots
• Shopify functions : Server-side logic for discounts, delivery, payment methods, and validation
• Branding API : Controlled styling of the hosted checkout

What you can build with them

• Pre-purchase upsell offers based on cart contents or customer data
• Custom banners with dynamic messaging (e.g., holiday delivery cutoffs)
• Custom input fields for gift messages or delivery instructions
• Checkout validation rules that block progress if conditions aren't met

5 Shopify Checkout Extension Apps

Shopify's Checkout Extensibility framework opened the door for apps that customize checkout without checkout.liquid hacks. Here are five of the best checkout extension apps merchants use in 2026, and what each one is best for.

1. Checkout Blocks & Customizer (by Marmeto)

This app lets you customize your Shopify Plus checkout with banners, promo code blocks, cart editing, custom fields, and smart checkout rules, all without coding. It's built on Shopify Checkout Extensibility.

Key features

• Banners and trust messages to boost conversions at checkout
• Cart editing directly in checkout, change quantity, remove items, swap variants
• Display available discount codes inside checkout to reduce drop-offs
• Collect GST/VAT, notes, or any custom fields during checkout
• Hide or reorder payment options with advanced checkout logic

Pricing: Free · Rating: 5.0 (10 reviews) · Trusted by USPA, Durex, Benetton, Birkenstock, and Love Beauty & Planet.

2. Checkout Components

A no-code platform for Shopify Plus checkout personalization. It lets merchants add upsells, cross-sells, and AI-powered product recommendations directly in the checkout flow. Conditional rendering by customer segment, cart value, Shopify Markets, and product tags means each shopper sees relevant offers.

3. ReConvert Post Purchase Upsell

ReConvert lets you build smart thank-you pages with upsell offers. After a customer pays, you can present additional items or special deals, it's effective because customers are already in a buying mindset.

4. Checkout Extensions Pro (MT)

Integrates with Shopify's Checkout Extensibility framework, enabling AI-powered upsells, dynamic offers, and custom checkout fields like gift messages. Includes trust badges, reviews, banners, custom UI blocks, post-purchase events, thank-you page surveys, and localized multi-language/multi-currency experiences. Also includes smart order validation to block fake orders.

5. Cartylabs Checkout Customizer

Works on Standard Shopify and Plus alike — core upsells, rewards bars, and trust badges work for every merchant, while features like custom branding profiles and unlimited extensions have Plus-only tiers. Offers a drag-and-drop editor, product upsells, free gift offers, shipping protection, custom banners, order notes, and full brand styling.

Which app should you choose?

If you're on Shopify Plus and want a free, no-code checkout customizer with cart editing, discount display, and payment logic, Checkout Blocks & Customizer is the strongest starting point. For post-purchase upsells on any plan, look at ReConvert. For deep AI personalization, Checkout Components or Checkout Extensions Pro may fit better.