Checkout Extension Checklist: Field Kit

Inventory

List every checkout customization: fields, messages, scripts and stylingAnything missing from the list is lost silently when the old checkout stops working.
Confirm your plan: full extension-based customization is generally PlusWithout Plus, standard checkout settings and apps are usually the realistic path instead.
Assign each item a tool: UI extension, Function, or brandingUsing the wrong tool leads to rebuilds, since each piece of the new model does one job.
Flag items with no direct equivalent and redesign them in the supported modelSome old hacks cannot be ported, and discovering that late stalls the whole migration.

Rebuild

Generate a checkout_ui extension with the Shopify CLIInterface edits outside supported extensions are the fragile kind this migration exists to end.
Render banners, upsells and messages at a defined checkout targetElements outside supported targets have no place to render and can break on platform changes.
Capture extra inputs with field components and attach them to the orderA field that is not saved to the order never reaches your team or fulfillment.
Apply colors, fonts and corner radius through the branding APICSS overrides are not the supported path and leave checkout off-brand or off-spec.
Enforce cart rules with a cart and checkout validation FunctionWithout validation, orders that break your business rules go straight through.

Ship and verify

Deploy with shopify app deploy and version extensions with the appChanges made outside a deploy are hard to review and hard to roll back.
Place each extension at its target in the checkout editorA deployed extension that is never placed does not appear to a single customer.
Test every rebuilt item end to end, including with Shop PayA customization that fails on one payment path costs orders you will not see in logs.
Compare the new checkout against the inventory, item by itemWithout a final check, a missing field or message is found by customers, not by you.

How to use this

Work the inventory first; it is the migration plan. A good result: every old customization is rebuilt or deliberately retired, each extension is placed and tested with Shop Pay, and nothing depends on checkout.liquid.

The rest of the kit is one step away.

Leave your email and the full kit opens right here, ready to work through or save as a PDF.

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