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Shopify Checkout Extension Development

Extension.
Development.

We build custom Shopify checkout extensions, Shopify functions that unlock the revenue your site can’t. From custom discounts, to A/B tests we validate high-performance checkout.

Shopify Checkout Interface
  • Smilepen
  • Sparebank
  • The Honest Kitchen
  • Vodafone
  • Yandy
  • CleanSkinClub
  • Biorenewal
  • Miniso
  • Isoclean
  • Gleeful
  • ThriveState
  • Rocka Nutrition
  • SKIOMusic
  • SIBERTHAL

Our Services

Custom pricing logic

Discount Functions

Tiered volume pricing, B2B rules, automatic bundles. Logic that runs natively on Shopify.

Branded checkout experience

Checkout UI Extensions

Trust badges, delivery promises, custom fields, upsell widgets. Your brand, your rules.

Validate before you launch

A/B Testing

We integrate with Intelligems. Test checkout changes against revenue, not guesses.

Custom pricing logic

Discount Functions

Tiered volume pricing, B2B rules, automatic bundles. Logic that runs natively on Shopify.

Branded checkout experience

Checkout UI Extensions

Trust badges, delivery promises, custom fields, upsell widgets. Your brand, your rules.

Validate before you launch

A/B Testing

We integrate with Intelligems. Test checkout changes against revenue, not guesses.

Custom pricing logic

Discount Functions

Tiered volume pricing, B2B rules, automatic bundles. Logic that runs natively on Shopify.

+60

Shopify Functions

+70

UI Extensions

+1257

A/B TESTS

+70

Projects

Visual Impact

The Difference is Measurable

See how a strategic checkout redesign transforms a generic input form into a branded conversion engine.

After - Optimized checkout
Optimized
Before - Standard checkout
Standard Shopify

Conversion Rate

3.8%+24%

Drag the slider to compare standard vs optimized experience

FOR SHOPIFY PLUS STORES

Your custom storefront

From your Figma designs to a live, high-performance Shopify Plus storefront. Custom product pages, collection layouts, and mobile-optimized experiences that convert.

Custom theme development

From your Figma designs to a fully functional Shopify Plus storefront. Advanced product pages, collection optimization, and custom theme extensions built to your specifications.

Recharge & subscriptions

Seamless Recharge subscription integration, Stay.ai review systems, and third-party app connections that enhance conversion and customer experience.

Performance optimization

Core Web Vitals optimization, image optimization, code splitting, and mobile-first responsive design that loads fast and converts better.

Custom theme development

Results delivered for leading brands.

Portfolio
Brand Logo
cleanskinclub.com
Portfolio
Brand Logo
fitnesssuperstore.com

Enterprise Checkout Engineering

Move beyond basic limitations. Scale without friction.

Your checkout is the most critical step in the funnel. We replace generic apps and unstable scripts with robust, custom-engineered Shopify Functions and UI Extensions designed for high-volume merchants.

  • ✓Conversion rate optimization (CRO) focused
  • ✓Seamless integration with ERP & OMS
  • ✓Optimized for Shop Pay & one-page checkout

Checkout Logic

Custom payment rules, dynamic shipping filtration, and sophisticated discount architectures running directly on Shopify's edge infrastructure.

Strategic Implementations

Engineered for Revenue

Advanced discount architecture

Implement complex tiered pricing, and specific logic that standard Shopify features can't handle.

DiscountsScript migrationPricing

Checkout UI customization

Inject trust signals, delivery promises, and custom fields directly into the checkout flow to reduce abandonment.

Trust profilesInformation captureBranding

Intelligems A/B testing

We partner with Intelligems to run scientific A/B tests on your checkout flow, validating every change against revenue metrics.

A/B testingData scienceValidation

Stop guessing. Start validating.

Don't deploy checkout changes based on hunches. We build testable hypotheses and implement rigorous A/B testing infrastructure to prove ROI before full rollout.

ControlVariant B

Interactive Showcase

Try customizing checkout

E
EXTFOX.
Show order summary
$249.83

Contact

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Email address or phone number

Delivery

Ship
Pickup in store
United States
William
Johnson
1234 Elm Avenue
Seattle
WA
98101

Shipping method

Standard shipping
$8.62
3 to 7 business days
Priority shipping
$11.58
2 to 4 business days

Payment

All transactions are secure and encrypted

Credit or debit card
VISA
AMEX
Card number
Expiration date (MM/YY)
Security code
Name on card
PayPal
PayPal

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Order summary

2
White kitchen chair
Curved seat with beech wood legs
$80.00
1
Side tables
Brown wooden side tables
$150.00
Subtotal$230.00
Shipping$8.62
Estimated taxes$11.21
Total$249.83
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Scroll through the page to see different checkout sections • Click buttons to add extensions in real-time

Engagement Models

Engineered for Revenue

We don't sell hours. We sell conversion outcomes. Choose the engagement that fits your scale.

Project Based

Checkout transformation

A complete overhaul of your checkout flow. We audit, design, and engineer custom functions to solve specific revenue blockers.

UX/UI redesign & branding
Custom validation & gateways
Smart upsell implementation
One-time delivery
Partner Program
Client preferred

Dedicated CRO team

We become your development arm. Continuous optimization, A/B testing of logic, and iterative improvements to drive long-term value.

Monthly A/B testing cycles
Quarterly strategy roadmaps
Priority integration support
Dedicated Slack channel

What our customers say

We're the #1 face towel on Amazon, but our Shopify offers were too slow and confusing. They rebuilt for speed, fixed our subscription flow, and our online revenue is now growing 18%.

They did more than deliver on time. Their proactive leadership and technical skill in delivering a really complex design from Figma ensured we launched our premium brand in weeks.

One custom app built by EXTFOX replaced five. It manages our discount tiers, runs 40% faster, and saved us $12K a year. We felt the difference!

Next-level communication and skill. They found our site was too confusing, simplified it, added trust, and fixed our conversion rate in a month. We are not burning money on ads anymore.

We needed an MVP to prove our model in 3 weeks, not a year. EXTFOX integrated our legacy portal with new payment systems and built the dashboard from scratch. They worked directly with my dev lead, and we launched in weeks.

CleanSkinClub testimonial
Biorenewal testimonial

FAQ

We create custom discount logic that applies tiered pricing directly in checkout. Whether it's 'Buy more, save more' or automatic bundle pricing, we build rules that increase order value without needing additional apps.
Yes. We add one-click upsells such as gift wrap, samples, or small add-on items directly in checkout. Customers can select them without leaving the flow, increasing both AOV and customer satisfaction.
Absolutely. We integrate with platforms like Intelligems or Launchpad. We can deploy two versions of a UI Extension (e.g., a 'Free Gift' bar vs. a 'Discount Code' helper) and split traffic between them. This lets you make data-driven decisions on which layout actually drives higher revenue
We go far beyond basic percentage off. We build complex logic such as Tiered Volume Pricing (Buy 5 Save 10%), Dynamic Bundling (Mix & Match), Free Gift with Purchase based on cart metadata, and VIP-specific pricing tiers that only activate for logged-in customers with specific tags.
Yes. We integrate Shopify checkout with external systems so you can validate inventory, check customer status, or display real-time delivery information before payment is completed.
Top-performing methods include smart upsells, free gift thresholds, loyalty point displays, and personalized recommendations. We help you implement the strategies most likely to increase your conversion rate and AOV.
Yes. We build subscription-specific checkout features such as product swaps, upgrade prompts, and custom messaging that improves retention and reduces support tickets.
Most checkout improvements launch within 2–4 weeks depending on complexity. Smaller enhancements can go live much faster, and we guide you through every step from design to deployment.
We treat checkout logic like mission-critical software. We use GitHub Actions for full CI/CD pipelines. Pushing to the 'main' branch automatically triggers a deployment via the Shopify CLI to your production environment, while pull requests generate preview URLs for staging and QA testing.
Yes, but with strict constraints. Since Shopify Functions are sandboxed, we use UI Extensions to fetch data asynchronously or deploy highly-available 'sidecar' APIs on Fly.io. This allows us to validate inventory or loyalty points in real-time (<100ms) without blocking the main checkout thread.
EXTFOX is an independent Shopify Partner and is not affiliated with or endorsed by Shopify Inc. We provide third-party development services for Shopify stores using Shopify's publicly available APIs and development tools. Our expertise comes from years of building and optimizing Shopify stores, but we operate independently from Shopify.

Speak with our team

Talk to a Shopify expert

Get a senior Shopify lead to review your store and send a clear plan for design, build, and growth.

Dusan Jovanovski

Dusan J.

Founder & CEO · 15+ years experience

Store review and opportunity map

Roadmap with scope, timeline, and priorities

Typical response time within 1 business day