Custom checkout logic,the way Shopify supports it.
checkout.liquid is gone. Everything custom in checkout now runs as a UI extension or a Shopify Function. We have built both for our own COD and partial-payment app, and we build them for merchants who need rules Shopify does not ship.
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In short
Byteinfy builds Shopify checkout UI extensions and Shopify Functions from $1,500: custom discounts, delivery and payment rules, upsells, address validation and COD logic that run inside Shopify's checkout rather than around it.
Last updated 5 September 2026 · Written by the Byteinfy team, which builds and operates SwipeReel and myBillPlease.
Who it is for
- Merchants who need tiered, cart-based or customer-based discounts
- Stores that must hide or reorder payment and delivery options by rules
- Brands adding upsells, gift messages or address validation at checkout
- Indian D2C brands running COD with fees, limits and OTP
What you get
- Shopify Functions for discounts, delivery customisation, payment customisation and cart validation
- Checkout UI extensions built with Shopify's extension targets, tested on checkout previews
- Configuration UI in the app admin so your team can change rules without a developer
- Migration off deprecated scripts and checkout.liquid customisations
How it works
- 1
Rules
We write the rules down in plain language and get them signed off before any code.
- 2
Build
Functions and extensions in a custom app on your store, demoed in checkout preview.
- 3
Ship
Enabled in a quiet window with a one-click way to turn it off.
Pricing
From $1,500
A single Function or checkout extension is $1,500 to $3,000. Bundled checkout programmes (COD rules, upsells, validation) are quoted together.
Questions we get asked
- Do we need Shopify Plus?
- Checkout UI extensions and most Functions work on all plans for the information and shipping steps; some payment-step customisations and checkout branding need Plus. We confirm on the call.
- Can you migrate our Shopify Scripts?
- Yes. Scripts run out in 2026; we rewrite them as Functions with the same behaviour.
Further reading
From the Byteinfy blog, where we write up what we learn doing this work.
Often paired with
Shopify App Development
Custom and public Shopify apps, built to pass App Store review the first time.
See what is includedFrom $1,500Shopify Conversion Rate Optimization
Product, cart and checkout fixes chosen from your data and measured against your conversion rate.
See what is includedFrom $1,500Shopify Integrations
Two-way sync between Shopify and your ERP, accounting, WhatsApp, shipping and CRM tools.
See what is includedNeed Checkout Extensions & Shopify Functions?Let us scope it this week.
A scoped plan, a timeline and a fixed quote, usually within 48 hours of the first call.