Promo code tracking is crucial for understanding what drives conversions, repeat purchases, and ROAS efficiency. With GA4 and GTM, you can track promo code usage during checkout in OpenCart using custom event parameters—even without modifying the checkout flow too heavily.

🧰 Prerequisites

Tool/Platform	Purpose
OpenCart 3.x or 4.x	eCommerce platform
Google Tag Manager	Installed sitewide
Google Analytics 4	Configured and linked with GTM
GA4 Admin Access	To register custom dimensions

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🎯 Goal

Track when a promo code is used in OpenCart, and pass that value to GA4 in the purchase event using a custom event parameter (e.g., promo_code).

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📋 Step 1: Extract Promo Code from Order (Server-Side)

OpenCart stores the promo/discount code in the order_total table when the coupon is applied. We’ll expose this to the front-end.

Edit success.twig:

In catalog/view/theme/YOUR_THEME/template/checkout/success.twig, add this snippet where you prepare your purchase event:

<script>
window.dataLayer = window.dataLayer || [];

dataLayer.push({
  event: 'purchase',
  ecommerce: {
    transaction_id: '{{ order_id }}',
    value: {{ total }},
    currency: '{{ currency }}',
    items: [
      {% for product in products %}
      {
        item_id: '{{ product.product_id }}',
        item_name: '{{ product.name }}',
        price: '{{ product.price }}',
        quantity: '{{ product.quantity }}'
      }{% if not loop.last %},{% endif %}
      {% endfor %}
    ],
    coupon: '{{ coupon if coupon else "" }}'  // This assumes your controller is passing `coupon`
  }
});
</script>

✅ If coupon isn’t available, you can extract it in the success() controller (catalog/controller/checkout/success.php) and pass it to the view:

$data['coupon'] = $this->session->data['coupon'] ?? '';

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🛠️ Step 2: Configure Data Layer Variable in GTM

1. Go to GTM > Variables
2. Add a new Data Layer Variable
   • Name: DLV - ecommerce.coupon
   • Data Layer Variable Name: ecommerce.coupon

This variable will pick up the promo code value from the purchase dataLayer.

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📨 Step 3: Modify GA4 Purchase Event Tag in GTM

1. Go to Tags > GA4 Event Tag > purchase
2. In Event Parameters, add:

Parameter Name	Value
coupon	{{DLV - ecommerce.coupon}}

3. Make sure ecommerce object is passed in the GA4 tag’s advanced configuration if using Enhanced Ecommerce-style payloads.

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🧾 Step 4: Register coupon as a Custom Dimension in GA4

1. Go to GA4 Admin > Custom Definitions
2. Click Create Custom Dimension
   • Dimension Name: Coupon Code
   • Scope: Event
   • Event Parameter: coupon

This will enable coupon reporting across GA4 Explorations and Reports.

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🔍 Step 5: QA the Data

Use the following tools to debug:

Tool	Use
GTM Preview Mode	Check if ecommerce.coupon is set correctly
GA4 DebugView	Ensure the coupon parameter appears
Dev Console	Run console.log(dataLayer) on thank you page

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🧠 Pro Tips

Tip	Benefit
Hash promo codes (optional)	Prevent competitor scraping in payloads
Send coupon with add_to_cart too	For abandoned cart recovery logic
Use coupon != null in GA4 filters	To build promo-based segments
Compare coupon usage vs. AOV	Identify discount-based value behavior

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📈 Strategic Uses

• Create GA4 audiences: Users who used SUMMER20 or any code
• Measure ROAS of specific promos
• Compare cart abandonment rate of promo vs. non-promo users
• Trigger remarketing based on coupon behavior

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