A checkout extension should solve a specific buying problem. More badges, fields and messages can add work for the customer as easily as they can help. Start with a requirement you can explain and test.
Check which surface your store can customise
Shopify's current documentation makes UI extensions on the information, shipping and payment steps available to Shopify Plus stores. Thank you and Order status extensions have different eligibility and are available on plans other than Starter. Confirm the exact target and plan before commissioning a feature. Checkout UI extension eligibility and checkout app extension surfaces are the source of truth.
An extension works through Shopify's supported targets and APIs. It is not permission to rewrite every part of checkout. Ask whether an existing, supported app can meet the requirement before commissioning a custom implementation.
Write one customer problem and one acceptance check
- Delivery information: customers need to understand a genuine dispatch constraint. The check is that the message matches the actual fulfilment rules for the relevant order.
- Business ordering: an approved buyer needs a specific reference. Establish whether it is necessary, how it reaches operations and what happens if it is missing.
- Product guidance: buyers need verified information before completing a purchase. Keep it accurate and accessible rather than using an unsupported trust claim.
- Post-order guidance: customers need a clear next step. Keep order details, support routes and tracking intact.
These are planning examples, not a report of features Sonder has delivered for an unnamed brand or a prediction of their commercial effect.
Keep UI, business rules and analytics distinct
Checkout UI extensions display supported interface elements. Shopify Functions implement supported commerce logic, such as eligible discount or delivery customisations. Pixels have a separate role in customer-event measurement. A visible message does not establish that the underlying rule or tracking works.
Shopify Scripts were deprecated on 30 June 2026, and published Scripts no longer work. Existing behaviour needs a compatible replacement. Shopify's Scripts notice documents that change. Also check the current upgrade notices for your Thank you and Order status pages in Shopify admin.
Do not assume Functions can never use external data. Shopify documents network access for specific functions and eligible arrangements, with enablement and restrictions. Confirm the supported API and failure behaviour for the actual requirement. Functions network-access guidance.
Agree safe testing before a live change
Use a suitable development or staging setup and a written scenario list. Include mobile, keyboard use, error states, applicable customer types, relevant discounts, delivery destinations and the way operations receives the order. Test only the payment methods and commercial arrangements in the authorised scope.
The store owner should review the finished experience and approve the release and rollback approach. This article is guidance, not authorisation to alter a live checkout, payment configuration or customer account.
Measure the intended outcome
Record the eligible traffic, baseline period and event definitions before a test. A message-view event is different from a checkout completion; an attributed order is different from incremental revenue. Note stock, promotions and tracking changes that affect the comparison.
Low-volume stores may not have enough observations for a reliable experiment. In that case, validate usability and correctness, state the limitation and avoid announcing a conversion uplift. There is no universal percentage gain from adding an extension.
For a clearer buying journey, explore conversion optimisation. For implementation scope, start with Shopify design and development or the checkout migration review. Ongoing Shopify work through Snowball follows agreed priorities and approvals.
Sources and review
Guidance checked on 7 October 2026. Features and commercial terms can change; confirm availability for your own account and scope. These planning checks are recommendations and do not replace an assessment of the actual implementation.