In WHMCS, addon merge fields allow you to dynamically insert specific information related to addons into email templates, making them more personalized and relevant to each recipient. Here are some common addon merge fields used in WHMCS:

1. Addon Details Merge Fields:

   • {$addon_id}: Unique ID of the addon.
   • {$addon_name}: Name of the addon.
   • {$addon_description}: Description of the addon.
   • {$addon_setup_fee}: Setup fee of the addon (if applicable).
   • {$addon_monthly_cost}: Monthly cost of the addon (if applicable).
   • {$addon_billing_cycle}: Billing cycle of the addon (e.g., "Monthly").

2. Product/Service Merge Fields for Addons:

   • These fields are related to the parent product/service to which the addon is attached.

   • {$service_id}: Service's unique ID.

   • {$service_product}: Name of the product/service.

   • {$service_domain}: Associated domain name.

   • {$service_domain_id}: ID of the associated domain.

   • {$service_regdate}: Registration date of the service.

   • {$service_nextduedate}: Next due date for payment.

   • {$service_nextinvoicedate}: Next invoice date.

   • {$service_billingcycle}: Billing cycle of the parent product/service (e.g., "Monthly").

3. Invoice Merge Fields for Addons:

   • These fields are related to the invoice for the addon.

   • {$invoice_id}: Invoice's unique ID.

   • {$invoice_num}: Invoice number.

   • {$invoice_date_created}: Date the invoice was created.

   • {$invoice_date_due}: Due date of the invoice.

   • {$invoice_total}: Total amount of the invoice.

   • {$invoice_subtotal}: Subtotal (excluding taxes and fees).

   • {$invoice_tax}: Tax amount.

   • {$invoice_balance}: Remaining balance due.

These merge fields allow you to customize email templates, invoices, and other communications related to addons. They make it possible to provide clients with detailed and relevant information about the addons they have purchased.

Remember to use these merge fields within the context of an email template associated with an addon or a parent product/service that has addons attached. This ensures that the merge fields retrieve and display the correct information for each recipient.