Scripts to Functions
Replace Shopify Scripts with Functions for discount, shipping and payment rules, decide who builds them, and know when an app costs less.
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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?
Custom Shopify Functions, code Shopify runs inside checkout, can be built by an agency or one in-house developer, but only Shopify Plus stores can run them as custom apps (shopify.dev, October 2026). The build is the small part. Hire whoever will change the rule and catch its failures, because Shopify's monitoring guide describes no merchant-facing alert (shopify.dev, October 2026).
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 Shopify Function, code Shopify runs at checkout, does not beat a discount app's three-year cost on one common rule. Deploi's Scripts-to-Functions record puts an illustrative three-year build cost, upkeep included, at $7,250 to $40,000, against $1,800 to $16,700 on apps (Deploi record, October 2026). So stacking closes the gap: the same record finds two or three app subscriptions close it around year three.
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, not fulfillment, is the Shopify Function to build first for most mid-market brands, because a discount rule maps straight to margin. Shopify documents seven kinds of Function, the custom code that runs at checkout. Only Shopify Plus stores can use custom apps that contain them (shopify.dev, October 2026). Validation goes first when your operations team fixes failed orders by hand.
What does it realistically cost to have an agency build a custom Shopify Function for one discount use case?
An agency-built Shopify Function, the custom code Shopify runs at checkout, has no published market rate for one discount rule. Shopify's partner listings show only partner-set starting prices or "Contact for pricing" (Shopify Partner Directory, October 2026). So the cost follows the rule: write it down, then compare quotes itemizing its data, discount collisions, configurability, cart size and alerting.
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: Shopify's App Store curates 34 discount apps built on Shopify Functions, the code Shopify runs at checkout. Several of them sell tiered volume discounts (Shopify App Store, October 2026). Dealeasy, a volume discount app, is free to 50 orders a month, then $9 to $29 USD monthly (Dealeasy listing, October 2026). No-code Function builders cover rules no single app ships.
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 clear Shopify Function brief answers 4 questions in plain words: what the rule reads and decides, how it meets other discounts, what happens when it fails, and who edits it. It needs no technical terms for Shopify Functions, the code Shopify runs at checkout. Shopify caps a Function's data query at 3,000 bytes (shopify.dev, October 2026), so the data the rule can request has a hard limit.