Auto Release
From WHMCS Documentation
The auto release module is designed to be used on products where there is no remote panel or automation to link to. It will return a successful dummy response to WHMCS when a creation is requested thereby allowing you to automate the switching to active of products and sending of the associated welcome email when there is no module.
To use it, simply choose the Auto Release module on the Module Settings tab of the product config area and then set the auto activation action to either immediately on order or on first payment, then save.