# Checkout rules

FunctionFlow AI · Function types · Updated October 3, 2026

> Stop checkout with a clear message when a cart breaks your rules: quantity limits, restricted products, PO Boxes and ZIP codes you can’t serve.

Checkout rules — *validation functions* — block checkout and show your message when a cart doesn’t meet your rules. In Shopify they’re listed under **Settings → Checkout**.

![A Restrict quantity action: per product, maximum 5, with the error message shown at checkout](https://help.fleekapps.com/media/functionflow-ai/validation-actions.webp "Maximum 5 units of each product, with a message customers see at checkout.")

## Actions

### Block checkout

Blocks checkout whenever the **When** conditions match and shows **Error message shown at checkout**. Use it for addresses, regions or carts you can’t accept — for example *Address is a PO Box is true* → *We can’t ship to PO Boxes. Please use a street address.*

> **Important:** With no conditions, **Block checkout** blocks every checkout. Always add conditions first.

### Restrict quantity

Limits how many units can be bought:

- **Limit** — **Per product** checks each cart item (each variant on its own); **Across the whole cart** adds up all matching items.
- **Minimum** and **Maximum** — leave one empty if you only need the other.
- **Products this limit applies to** — all products, or only those matching a filter.
- **Message** — you can use `{max}`, `{min}` and `{quantity}`: *You can buy at most {max} of each item.*

### Restrict products

Blocks checkout when the cart contains products matching **Restricted products** — for example products tagged *wholesale-only*. Add conditions to decide who the restriction applies to, such as *Customer tags has none of wholesale*.

## Examples

| Goal | When | Then |
|---|---|---|
| Max 5 per product | — | Restrict quantity → per product, maximum 5 |
| Minimum order | Cart total is less than 50 | Block checkout → *Orders start at $50.* |
| No PO Boxes | Address is a PO Box is true | Block checkout |
| Blocked ZIP codes | ZIP / postal code is any of 96799, 96960 | Block checkout → *Sorry, we can’t deliver to this ZIP code.* |
| Sign-in required | Customer is logged in is false | Block checkout → *Please sign in to complete your order.* |
| Wholesale-only products | Customer tags has none of *wholesale* | Restrict products → tagged *wholesale-only* |

## Write helpful messages

Customers see your message at checkout and need to know how to fix the problem. Say what’s wrong and what to do — *“We can’t ship to PO Boxes. Please use a street address.”* — in up to 250 characters.

## If a validation can’t run

In [Settings](https://help.fleekapps.com/functionflow-ai/settings/), **Block checkout if a validation function errors** decides what happens in the rare case a validation fails to run. **Off** (recommended) lets checkout continue; **On** blocks it until the function runs successfully. The setting is applied to each checkout rule when it’s published, so publish your rules again after changing it.

## Things to know

- Checkout rules don’t change orders, so they don’t appear in [Analytics](https://help.fleekapps.com/functionflow-ai/analytics/).
- Shopify allows up to **25 checkout validations** per store.
- Address conditions only apply once the customer has entered an address.

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Source: https://help.fleekapps.com/functionflow-ai/checkout-rules/