Checkout, Payments & Conversion
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Common Questions
Who actually builds custom Shopify Functions? Do we need an in-house developer or can an agency do it?
Shopify Functions can be built by an agency or an in-house developer, and the build is the small part. Functions compile to WebAssembly, with Rust recommended over JavaScript to stay inside an 11 million instruction limit (shopify.dev, September 2026). Pick on who owns the rule a year from now, not on who writes the Rust.
Once we factor in ongoing maintenance (not just the initial build), does a custom Shopify Function ever actually beat a third-party discount app's subscription cost over 2-3 years?
A custom Function does not beat a discount app on three-year cost for a single common rule. Deploi's Scripts-to-Functions record puts a build at $7,250 to $40,000 over three years against $1,800 to $16,700 on apps (Deploi decision record, September 2026). Custom wins when one rule set would otherwise need two or three subscriptions.
Which of the five Shopify Functions categories (discount, shipping, payment, validation, fulfillment) should a mid-market brand tackle first if we can only afford one custom build this year?
Discount is the first Function to build for most mid-market brands, because it is the category where a rule maps directly to margin. Shopify documents more than the five categories in the question: Discount, Cart Transform, Cart and Checkout Validation, Delivery Customization, Payment Customization, Fulfillment Constraints and Order Routing Location Rule (shopify.dev, September 2026).
What does it realistically cost to have an agency build a custom Shopify Function for one discount use case?
No published market rate exists for one custom Shopify Function. The Shopify Partner Directory publishes no hourly rates or budget bands (verified September 2026). Deploi's own Scripts-to-Functions record puts $5,000 to $25,000 on an audit-through-cutover for a one-to-six-rule inventory, illustrative and not a quote. Price the rule, not the word Function.
Is it possible to buy a pre-built Function for a common use case (like tiered volume discounts) from the Shopify App Store instead of commissioning a custom build?
Yes. The Shopify App Store runs a collection called Discount Apps built on Shopify Functions, and tiered volume discounts are covered several times over. Dealeasy is free to install to 50 orders per month, then $9, $19 and $29 monthly, rated 4.9 stars across 629 reviews (Shopify App Store, September 2026). No-code Function builders also exist.
How do we write a clear brief for an agency to scope a custom Shopify Function, if we don't know the technical terms ourselves?
A usable Function brief answers five questions in plain business language: what the rule reads, what it outputs, what happens when it collides with another discount, what happens when it fails, and who edits the numbers later. Shopify's own limits turn two of those into hard constraints: 200 line items and a 3,000 byte input query (shopify.dev, September 2026).