Shopify AppsDevelopment
Checkout Guardian Pro
Shopify app in development that enforces checkout rules, validates purchases and applies operational safeguards.
- Shopify
- Checkout
- Validation
- Functions
Ecommerce engineering
Shopify Functions
Shopify Functions Development
Some checkout rules cannot be an app or a script — they have to run inside Shopify's own checkout. Functions are how. We build them, test the edge cases, and configure them through metafields so the merchant can change the numbers without a deploy.
Overview
Logic that runs where the decision is made
A Function is a small piece of logic you hand to Shopify, which Shopify executes at the moment it prices a cart, validates a checkout, or picks a delivery option. It runs on their infrastructure, inside the checkout, with no network call to you — which is exactly why it can do things an app cannot.
The interesting part is rarely the happy path. It is what the rule does to a gift card, to a cart of eleven when the pricing was written for one, to a product that should have been excluded, and to a discount that meets another discount. Those are the cases we write tests for, because they are the ones that reach a customer.
- Runs inside checkout
- No callback to your server, no latency budget, no outage of yours that can break a purchase. Shopify executes it.
- Configurable without a deploy
- Rates, thresholds and exclusions live in metafields, so changing a fee is an admin edit rather than a release.
- Quantity-safe by construction
- Per-unit versus per-line is the most common Function bug in production. We write the tests that catch it before a customer does.
What's included
What the engagement covers
- Discount Functions — order, product and shipping discounts
- Cart Transform — added fees, bundles and line-item changes
- Checkout validation — blocking a purchase that must not proceed
- Delivery and payment customization rules
- Metafield-driven configuration the merchant can edit
- Unit tests over the exclusion and quantity edge cases
Technologies
- Shopify Functions
- Cart Transform API
- Shopify Plus
- Rust
- GraphQL
- Metafields
Use cases
Where this helps most
A fee that applies to some items and not others
An import fee, a handling charge or a surcharge that must skip gift cards, specific products or tagged collections, and must be right at any quantity.
A discount your promotions engine cannot express
Tiered, conditional or combination logic that the native discount types do not cover.
A checkout that must refuse
A cart that should not be allowed to complete — restricted items, incompatible combinations, a delivery region you cannot serve.
Process
How we deliver
Write the rule down
In prose first: what it applies to, what it excludes, what happens at quantity two, and what happens when it meets another discount. Ambiguity here becomes a bug later.
Build the Function
Implemented against the Function API for its target, with configuration read from metafields rather than compiled into the logic.
Test the edges
Gift cards, excluded tags, mixed carts, high quantities, and the interaction with existing discounts — each as a test rather than a manual check.
Deploy and observe
Released through your app, verified against real carts in a development store, then watched on the first live orders it touches.
FAQ
Shopify Functions Development
Do we need Shopify Plus?
For most Function targets, yes — Cart Transform and checkout validation in particular are Plus features. Some discount Functions are available more broadly. We check your plan against the specific target before scoping, rather than after.
Can the merchant change the rule afterwards?
The values, yes — that is what the metafield configuration is for, so a rate or a threshold is an admin edit. Changing the shape of the rule itself, like adding a new kind of exclusion, is a code change and a deploy.
How do Functions interact with our existing discounts?
That is one of the first things we pin down, because it is where the surprises live. Combination behaviour is explicit in Shopify's model, so we decide it deliberately and test the combinations rather than discovering them in production.
Rust or JavaScript?
Shopify's Function runtime targets WebAssembly and Rust is the best-supported path, which is what we usually reach for. The choice affects build tooling, not what the Function is allowed to do.
Proof
Where we have done this
Real projects that used this capability. Every one shipped.
Shopify FunctionsIntegrated
Private Discount Function
Private Shopify discount function that applies conditional discounts based on product eligibility and configurable business rules.
- Shopify Functions
- Discounts
- Checkout
- Business Rules
Shopify FunctionsConfidential
Private B2B Purchase Gate
Private B2B purchase gate that authorizes eligible customers and enforces purchase restrictions during checkout.
- B2B
- Checkout Validation
- Shopify Functions
- Access Control
Go deeper
Written from the work
- When a Shopify Function is the right toolA Function runs inside Shopify's checkout, which is both its power and its cage. What each target can do, what really needs Plus, and when to build something else.
- Building a Cart Transform that gets quantities rightA fee that is right at quantity one and wrong at quantity eleven is the signature bug of this work. Here is the order of operations that avoids it.
- Shopify is moving checkout validation off the clientJanuary made blocking opt-in. July deprecated the API that did the blocking. Read together they are one decision, and the reason is express wallets.
Let's build
Ready to start with Shopify Functions development?
Tell us about your store and your goals. We'll come back with a clear, honest plan and a transparent quote.