Testing On Checkout_completed

checkout_completed is the event that matters most and the best one to test against — it fires once when a purchase completes (usually on the Thank you page) and carries the full order. Walk it end to end.

The catch: Preview won't reach checkout

This surprises everyone. GTM Preview / Tag Assistant cannot connect to the checkout — it can't see into the pixel sandbox where your GTM and dataLayer live. So you test a different way:

• Use the Shopify Pixel Helper to confirm the events reach the sandbox without errors.
• Open DevTools Console and switch the JavaScript context (the frame dropdown) to the pixel sandbox, then inspect window.dataLayer there.
• Place a real test order (a 100%-off discount code is the usual trick) and watch the checkout_completed push appear.

Read every variable from the push

When the event fires, you should see an object like this in the sandbox dataLayer. Each key is a value you'll read with a Data Layer Variable in GTM:

{
  event: "checkout_completed",
  orderId: "gid://shopify/Order/5123456789",  // → transaction_id
  currency: "USD",                            // → currency
  value: 129.0,                               // totalPrice.amount → value
  subtotal: 110.0,                            // subtotalPrice.amount
  shipping: 9.0,                              // shippingLine.price.amount → shipping
  tax: 10.0,                                  // totalTax.amount → tax
  client_id: "abc123...",                     // Shopify clientId
  token: "a1b2c3...",                         // unique checkout token
  timestamp: "2026-06-06T14:05:00Z"
}

Wire it up in GTM

• Variables: a Data Layer Variable for each key — orderId, currency, value, shipping, tax, and your reshaped items.
• Trigger: a Custom Event trigger with event name checkout_completed — it must match the event key in the pixel exactly.
• Tag: a GA4 Event tag named purchase, mapping transaction_id, value, currency, shipping, tax and items to those variables.
• Verify: confirm each parameter shows the real order value, and that it fires once.