In a suit under this subchapter, a seller or retailer may be served with process as provided by the rules of civil procedure or by service on an agent or clerk in this state employed by the retailer or seller in a place of business in this state maintained by the seller or retailer. If process is served on the agent or clerk of the retailer or seller, a copy of the process shall forthwith be sent by registered mail to the retailer or seller at his principal or home office.