In order to assign custom email templates to products you must first create at least one email template. Once that is done you can navigate to Settings > Custom Communications. In the top left will be an Assign button. Click this to assign email templates to products.
Listed will be all pre-activity, post-activity, and post-purchase emails created in the system. Select the desired products for each email so that the system knows which email(s) to send for each product. Note that scheduled activities are the only products that can be assigned to pre-activity or post-activity emails.
When finished select Save Changes. All existing and new reservations that fall within the criteria on the email template settings will start receiving the emails if the product was attached.