If your marketing flow includes separate landing pages (e.g., on Unbounce, Webflow, or custom domains) that redirect users to your osCommerce store, standard GA4 tracking won’t capture the user session properly — leading to broken attribution and missed conversions.

This guide walks you through implementing cross-domain tracking using Google Analytics 4 (GA4) and Google Tag Manager (GTM), ensuring full-funnel visibility from click to purchase.

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🎯 Why Cross-Domain Tracking Matters in osCommerce

Without Cross-Domain Tracking	With Cross-Domain Tracking
Sessions split between domains	Single, unified session across domains
“Direct” or “(not set)” traffic sources	Correct attribution to source/medium
Inaccurate conversions in Google Ads	Accurate ROAS and assisted conversions
Incomplete funnel in GA4	Full view of user journey

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✅ Prerequisites

• GA4 and GTM installed on:
  • Your landing page domain (e.g., promo.example.com)
  • Your osCommerce store (e.g., shop.example.com)
• Both domains are under your control
• GA4 configuration tag already firing on both
• Shared GTM container or synced setups

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🧭 Step 1: Configure GA4 Cross-Domain Measurement in GTM

🔧 Edit Your GA4 Configuration Tag

1. In GTM, go to Tags → GA4 Configuration
2. Click to Edit Tag
3. Expand Fields to Set
4. Click Add Row

Field Name	Value
linker	true
linker_domains	shop.example.com

(On the store side, use promo.example.com instead if needed)

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🧪 Example GA4 Config Field Setting in GTM:

[
  { fieldName: "linker", value: true },
  { fieldName: "linker_domains", value: "shop.example.com" }
]

This ensures GA4 passes the _gl parameter between domains to stitch sessions together.

🌍 Step 2: Enable Auto Link Decorator in GTM

In the GA4 Configuration Tag:

1. Expand Advanced Settings → Tag Sequencing
2. Ensure this tag fires before other event tags

Alternatively, enable automatic link decoration in the GA4 config itself:

• Add a trigger to listen for clicks and pass the decorated link using GA4’s linker.

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🔗 Step 3: Modify Outbound Links on Landing Pages (if needed)

GA4 should auto-decorate links pointing to your osCommerce store. But you can also manually add linker parameters using GTM or JavaScript.

✅ Example: Manual Link Decoration (if JS injection is possible)

<script>
window.addEventListener('DOMContentLoaded', function () {
  var links = document.querySelectorAll('a[href*="shop.example.com"]');
  links.forEach(function (link) {
    link.href = gtag.get('linker').decorate(link.href);
  });
});
</script>

🧾 Step 4: Enable Cross-Domain Measurement in GA4 Admin

1. Go to GA4 Admin → Data Streams → Your Web Stream
2. Scroll to Configure tag settings
3. Click “Configure your domains”
4. Add:
   • promo.example.com
   • shop.example.com
5. Save changes

This tells GA4 which domains to treat as the same session context.

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🧠 Step 5: Test Cross-Domain Flow

✅ Use GTM Preview Mode:

1. Navigate to the landing page
2. Click a CTA link to go to the osCommerce site
3. Ensure the URL contains a _gl parameter (e.g. ?_gl=1*abc123...)
4. Open DevTools → Application → Cookies → Check that GA4 client ID remains consistent across domains

✅ Use GA4 DebugView:

• Confirm all events (from landing page to purchase) flow into a single session

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🧪 Bonus: Push Purchase Data on osCommerce Checkout

In checkout_success.php:

<script>
window.dataLayer = window.dataLayer || [];
dataLayer.push({
  event: "purchase",
  ecommerce: {
    transaction_id: "<?= $order['id']; ?>",
    value: <?= $order['total']; ?>,
    currency: "USD",
    items: <?= json_encode($order['products']); ?>
  }
});
</script>

Create a GA4 Event tag in GTM for purchase to finalize the conversion.

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🔐 Step 6: Consent Mode & Privacy Compliance (if needed)

• Use Consent Mode v2 in GTM if users are in GDPR/CCPA regions
• Ensure _gl parameters are passed only after consent, or anonymize link decoration

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📊 Post-Setup: How to Use the Data

In GA4:

• Use Exploration Reports to visualize cross-domain sessions
• Filter by page_location or page_referrer to see journeys
• Use Attribution > Conversion Paths for assisted conversions

In Google Ads:

• Use GA4-linked conversions
• Create remarketing audiences based on full-funnel behavior

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✅ Summary

With this setup, you’ve enabled accurate cross-domain tracking between your landing pages and osCommerce store, enabling:

• ✅ Unified user journey across domains
• ✅ Accurate session attribution for Google Ads
• ✅ Enhanced conversions and retargeting visibility

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📁 Optional Add-ons

• 🔄 Setup server-side GTM to capture linker data with higher accuracy
• 🔁 Use GA4 audiences to retarget product viewers via Google Ads
• 📉 Create Looker Studio dashboards with full funnel + attribution paths

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