Why Your GA4 E-Commerce Data Doesn't Match Shopify: Solving Conversion Leakage

// Inside Shopify Customer Events Custom Pixel
analytics.subscribe("checkout_completed", (event) => {
  const checkout = event.data.checkout;
  const orderId = checkout.order ? checkout.order.id : checkout.token;
  
  // Guard against reload duplicate executions
  const processedOrders = JSON.parse(localStorage.getItem("_ad_processed_orders") || "[]");
  if (processedOrders.includes(orderId)) {
    console.log("Order " + orderId + " already processed. Suppressing duplicate purchase event.");
    return;
  }
  
  processedOrders.push(orderId);
  localStorage.setItem("_ad_processed_orders", JSON.stringify(processedOrders.slice(-50)));

  // Forward clean purchase event to server endpoint
  fetch("https://data.analyticsdev.com/event", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({
      event: "purchase",
      ecommerce: {
        transaction_id: orderId,
        value: checkout.totalPrice.amount,
        currency: checkout.totalPrice.currencyCode,
        tax: checkout.totalTax ? checkout.totalTax.amount : 0,
        shipping: checkout.shippingLine ? checkout.shippingLine.price.amount : 0,
        items: checkout.lineItems.map(item => ({
          item_id: item.variant.id,
          item_name: item.title,
          price: item.variant.price.amount,
          quantity: item.quantity
        }))
      }
    })
  });
});