Introducing Checkout Functions: Customize Every Checkout Step

Checkout Functions give merchants a visual rule builder and WASM runtime to control discounts, shipping, payments, and validation at checkout.

Cartly Team

Engineering & Product

Today we're releasing Checkout Functions — a powerful extensibility engine that lets merchants customize every step of the checkout process without writing code.

Rule-Based Logic

Build conditions like "cart subtotal > $100" and combine them with actions like "apply 10% discount" or "hide Cash on Delivery". The visual Rule Builder supports AND/OR logic, nested condition groups, and 12+ comparison operators.

Four Function Types

Discount Calculate — Stack rule-based discounts on top of discount codes
Shipping Filter & Sort — Hide, rename, and reorder shipping rates dynamically
Payment Customization — Show or hide payment methods based on cart context
Checkout Validate — Block checkout with a custom error for cart minimums, restricted items, or address rules

WASM Runtime for Developers

Need more than rules? Write a WebAssembly function in any language that compiles to WASM. Upload it via the API and the wazero sandbox runs it with a 50ms timeout — safe, isolated, and fast.

Checkout Branding

The new Checkout Branding editor lets you match your checkout to your brand: set primary colors, button styles, corner radius, and typography without touching code.

Extension Points

App developers can now register UI extensions at 8 checkout points — inject banners, custom fields, or loyalty widgets using our Remote UI component system.