If you have already created a Product you can then customize the Welcome Email that is used for each product. In this guide, you can learn how to assign a custom Welcome Email notification to a Product/Service in WHMCS.
Assign Welcome Email to Product
Navigate to Setup > Products/Services > Products/Services.
Click on the Edit icon to the right of the Product you would like to customize the Welcome Email for.
Under the Details tab, select the Custom Welcome Email Notification you created from the Welcome Email drop-down menu.
Click the Save Changes button.
Congratulations! Now you know how to assign a custom welcome email template to a product. Now your WHMCS can be configured to send a customized welcome email depending on which one is configured for the Product/Service.