+60
Shopify Functions
+60
Shopify Functions
Shopify checkout extensions built to convert
Custom Checkout UI extensions and Shopify functions for Plus brands. Build custom checkout logic, remove friction, and test what actually moves conversion.
+70
UI Extensions
+70
UI Extensions
+1,900
A/B Tests
+1,900
A/B Tests
+60
Shopify Stores
+60
Shopify Stores
Shopify brands we've built for
Checkout extensions and stores we've optimized.
Eddi
Express checkout
Cart upsells and Shop Pay friction removed
What we build
What can we change inside Shopify checkout?
We build custom checkout experiences using Shopify's native extensibility. UI Extensions for custom interfaces and Functions for business logic that runs at the edge.
Checkout UI Extensions
Upsells, delivery messaging, custom fields, validation, B2B information, trust content and contextual checkout experiences.
Shopify Functions
Custom discounts, delivery rules, payment customization, validation and pricing logic running within Shopify's checkout infrastructure.
Advanced Shopify Plus checkout
Custom logic across information, shipping and payment steps, plus B2B and complex checkout workflows.
Interactive Showcase
Try customizing checkout
E
EXTFOX.Show order summary
$249.83
Contact
Log inEmail address or phone number
Delivery
Ship
Pickup in store
United States
William
Johnson
1234 Elm Avenue
Seattle
WA
98101
Shipping method
Standard shipping
3 to 7 business days
Priority shipping
$11.582 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
Remember me
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
Scroll through the page to see different checkout sections • Click buttons to add extensions in real-time
Where checkout starts costing revenue
Where Shopify checkout starts working against growth
Discount logic has outgrown Shopify settings
Bundles, tiers, customer groups or conditions now require multiple apps or manual workarounds.
Customers leave checkout with unanswered questions
Delivery timing, eligibility, product information or trust concerns appear exactly where purchase intent is highest.
Apps control business-critical checkout logic
Multiple checkout apps create recurring cost, dependencies and logic your team doesn't own.
Changes ship without knowing what worked
Checkout modifications go live, but nobody can attribute the conversion or revenue impact.
Our approach
How we build and validate checkout changes
We start with the constraint, then choose configuration, an existing app, or custom extensions only when that is the right answer.
- 01
Find the checkout constraint
Review drop-off, business requirements and existing checkout configuration.
- 02
Define the solution
Decide whether Shopify configuration, an existing app, UI Extension or Function is the right answer.
- 03
Build and QA
Implement against realistic checkout scenarios, edge cases and existing integrations.
- 04
Launch and measure
Deploy safely, track conversion and test changes where measurement makes sense.
Fit
When custom checkout development is worth it
We'll recommend focusing on other areas when checkout isn't the bottleneck. Extensions make sense when checkout performance is limiting your conversion potential.
Focus elsewhere when
Traffic quality is the real conversion issue
Poor traffic quality, weak product-market fit, or unclear value propositions hurt conversion more than checkout friction. Fix the funnel before optimizing checkout.
Shopify developmentYour checkout already converts well
If checkout conversion rates are already strong, other areas of your funnel probably offer better ROI for optimization efforts. Don't optimize what's working.
Basic theme improvements would have bigger impact
Product pages, navigation, search, and collection layouts often impact conversion more than checkout optimization. Start with the biggest bottlenecks first.
Plus storefronts
Checkout extensions make sense when
You can measure checkout conversion drop-off
Clear data showing where customers abandon checkout and specific friction points that extensions can address. Data-driven optimization opportunities.
You need custom business logic in checkout
Unique discount rules, shipping logic, upsell opportunities, or validation requirements that standard checkout can't handle natively.
Standard checkout no longer supports the business rules
Your B2B, delivery, payment, validation or customer-specific logic has outgrown Shopify's standard configuration.
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%.
Engagement
How can we work on your checkout together?
Whether you need a checkout audit, specific extensions built, or ongoing conversion optimization, we have engagement shapes that fit different stages of your checkout journey.
Checkout audit & roadmap
Scoped after reviewing your store.
Who it is for
For teams that know checkout has problems but don't yet know which changes justify custom development.
What is included
- Checkout performance analysis and conversion opportunity mapping
- Extension strategy and technical feasibility assessment
- Implementation roadmap with clear priorities and timeline estimates
How it ends
Ends with a clear optimization strategy and detailed plan for the highest-impact checkout improvements.
Checkout extension development
From $3,999 USD
Scoped to specific extensions and functions.
Who it is for
For defined UI Extensions, Shopify Functions and checkout integrations.
What is included
- Custom extension development with conversion-focused design
- Testing, performance optimization, and deployment with monitoring
- A/B testing setup and conversion tracking implementation
How it ends
Results in live checkout extensions with measurable conversion impact and clear performance metrics.
Checkout growth
From $3,500 per month USD
Monthly. Three months minimum for meaningful results.
Who it is for
For ongoing experiments, optimization and new checkout capabilities.
What is included
- Sprint planning focused on conversion goals and revenue impact
- New extension development and existing extension optimization
- A/B testing, conversion tracking, and rapid iteration on checkout improvements
How it ends
Runs as an ongoing partnership with regular conversion improvements and documented ROI measurement.
Not sure which type of checkout work you need? Book a meeting with the team.
Not ready to book?
Complementary services
When checkout isn't the only bottleneck
Checkout optimization often pairs with other Shopify development work. These services frequently complement extension development for complete conversion optimization.
Let's meet
Book directly with Dusan J., Founder at EXTFOX. We'll review the checkout constraint, whether it needs custom development, and what we'd test first.
Looking to scale revenue on Shopify?
Pick a time that works for you. You'll get instant confirmation and a calendar invite.
Prefer to call? (862) 259-6969
Not ready to book?
FAQ
Checkout UI Extensions let apps add functionality at Shopify-defined points in checkout. Shopify Functions customize backend checkout logic such as discounts, delivery, payment and validation.
Shopify Functions customize backend checkout logic such as discounts, delivery rates, payment methods and validation. UI Extensions add interface and functionality at Shopify-defined points in checkout, including fields, upsells, messaging and trust content. Functions change checkout logic; UI Extensions add checkout interface and functionality. Checkout styling itself is largely controlled through Shopify's checkout editor and Branding APIs.
For custom UI Extensions on the information, shipping and payment steps, yes. Shopify Plus also unlocks custom-app access to Shopify Functions and additional checkout capabilities. Some checkout extensibility is available on lower plans, particularly post-purchase account surfaces and functions delivered through public apps.
Results depend entirely on the bottleneck. We measure checkout changes against conversion, revenue per visitor, AOV and abandonment rather than promising a fixed uplift.
Timeline depends on complexity. Simple UI Extensions might take 2-3 weeks from design to deployment. Custom Functions with complex business logic typically require 4-6 weeks. Comprehensive checkout optimization with multiple extensions and A/B testing can take 6-12 weeks. We provide detailed timelines during discovery based on your specific requirements.
Extensions need minimal maintenance since they run on Shopify's infrastructure. However, business logic updates, performance optimization, and new feature development require ongoing attention. We provide retainers that include monitoring, updates, and continuous optimization based on conversion data and business needs.