Direct answer: Take the most unusual rule in your store and make the proposed platform prove it in a test account on the same plan and checkout technology you intend to buy. A slide saying “customizable checkout” is insufficient. The proof needs the buyer-facing step, validation, final order data, staff workflow, and failure path.

Suppose a gift store promises delivery on a chosen day, but certain products cannot ship on Mondays. The rule must appear at the right moment, reject an impossible date, carry the approved date into the order, and remain clear on mobile. Whether that happens through native settings, an app, an extension, or custom code matters more than a broad “Shopify versus WooCommerce” ranking.

Start with one rule and its observable result

Write the rule without naming a platform: “For the selected product and destination, show only valid delivery dates; the confirmed order must retain the chosen date and fulfillment staff must see it.” Then add exceptions: mixed cart, changed address, unavailable slot, browser back, and a customer who retries. A passing demo should use those cases, not just the easy default product.

Ask each proposer for a short feasibility receipt:

  • The exact plan, checkout type, extension/app, and version used in the test.
  • The screen where the buyer sees and changes the field, including mobile.
  • The rule that rejects an invalid choice and its customer-facing explanation.
  • The order field or metadata the warehouse will receive.
  • The owner of the extension and what happens when the app or API is unavailable.
  • The test record and limitations: what worked, what requires another plan or service, and what has not been tried.

This is a buyer's proof request, not a claim that every delivery-date rule needs custom code. The example can be replaced with your real hard requirement: split shipping, subscription eligibility, tax-exempt buyer data, or a product configuration that changes the total. Confirm legal and tax requirements separately where applicable.

Check the actual extension boundary

Shopify's current checkout technology guide describes different extension mechanisms and their availability by plan. In particular, checkout UI extensions on the information, shipping, and payment steps are listed for Shopify Plus, while some post-purchase or order-status extension points have different availability. That difference can make two demos look comparable when they are proving different parts of the journey. Confirm the current plan and target before accepting a proposed workaround.

WooCommerce's Cart and Checkout Blocks documentation describes a block extension route. Its hook-alternatives reference shows that legacy hooks have different levels of support in block checkout. An old plugin working in a classic shortcode checkout does not, by itself, prove it works in the block setup the agency plans to ship. Test the exact plugins and theme together.

Neither vendor document says which platform is best for your store. It defines what must be verified for a particular requirement. If a proposer suggests a headless or custom front end, ask them to list the payment, inventory, fulfillment, and maintenance work that moves into the custom scope.

Decide after the proof, not after the feature list

If native settings pass the rule, do not buy a custom build solely for that feature. If an app or extension passes, price its license, owner, update path, and failure behavior in the proposal. If the rule cannot pass on the proposed plan, change the scope, plan, workflow, or platform before a deposit turns the assumption into a delivery problem. Keep the failed test as evidence, not as an embarrassment to hide.

The payment and order-state acceptance guide tests what happens after payment attempts; this proof tests whether an unusual checkout rule is feasible before platform selection. The wrong-variant cart check is a later order-integrity test for a particular product selection. ARCHV's website work can assess commerce fit separately, with platform behavior and required integrations named in the proposal. Bring the hardest checkout rule to the website estimate.