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How to Sync Custom WooCommerce Checkout Fields to MailChimp

16 threads Sep 16, 2025 Plugin → Mailchimp for woocommerce

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A common challenge for WooCommerce store owners is capturing important customer data, like a birthday or phone number, during checkout and having that information sync automatically to their Mailchimp audience. While the 'Mailchimp for WooCommerce' plugin handles core fields like email and name out of the box, custom fields require additional configuration.

Why Custom Fields Don't Sync Automatically

The plugin is designed to sync a predefined set of standard WooCommerce customer and order data. It does not automatically detect or map custom fields added to your registration or checkout forms. This is a known limitation, and a feature request to natively support this has been under consideration by the 'Mailchimp for WooCommerce' team for some time.

The Solution: Using Custom Code Hooks

The primary method for sending custom data is to use a WordPress filter hook provided by the plugin. This requires adding custom code to your site, typically in your child theme's functions.php file or via a code snippets plugin.

Step 1: Prepare Your Mailchimp Audience

Before writing any code, you must first create a corresponding custom merge tag in your Mailchimp audience for the data you want to collect (e.g., BIRTHDAY, PHONE). Note the exact merge tag name (e.g., MMERGE5) as you will need it for the code.

Step 2: Add the Custom Code

The most commonly referenced filter in the plugin's community discussions is mailchimp_sync_user_data. This filter allows you to modify the customer data array before it is sent to Mailchimp.

Example: Syncing a Birthday Field
If you have stored a customer's birthday in user meta under the key birthday, you could use code like this:

add_filter( 'mailchimp_sync_user_data', function( $data, $user ) {
    // Retrieve the custom field value
    $birthday = get_user_meta( $user->ID, 'birthday', true );
    
    // Map it to your Mailchimp merge tag
    if ( !empty( $birthday ) ) {
        $data['MMERGE5'] = (string) $birthday; // Cast to string to avoid errors
    }
    
    return $data;
}, 10, 2 );

Important Considerations:

  • Data Format: The format of the data you send (e.g., DD/MM/YYYY vs. MM/DD/YYYY) must match the format expected by the field you created in your Mailchimp audience, otherwise the sync will fail for that field.
  • Data Source: The code must correctly target where your custom field data is stored, whether it's in user meta, post meta for the order, or a custom table. The above example uses user meta.
  • Error Checking: Adding logging can help troubleshoot why data might not be sending. As suggested in one thread, you can add mailchimp_log('trace', 'custom_fields', $data); inside your function to see what data is being processed.

Troubleshooting Common Issues

  • Code Doesn't Work: Double-check the merge tag key. Ensure your code is running on a site with the plugin active and that there are no PHP syntax errors.
  • Field Syncs But Format is Wrong: Verify the data format in your WooCommerce database matches the format required by the Mailchimp field.
  • Data is Not Stored on the User Object: If your custom checkout field data is saved to the order instead of the user, you may need to explore other hooks that provide access to the order object during the sync process.

Official Documentation and Resources

The 'Mailchimp for WooCommerce' team maintains a wiki that includes a section on Custom Merge Tags. This resource provides further technical context and examples for developers.

For complex use cases, such as adding an entirely separate person (like a gift recipient) as a new subscriber, the solution likely requires significant custom development beyond simple filters and may involve using the Mailchimp API directly.

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