24. Any person operating a commercial establishment who contravenes a provision of section 18, 21 or 22, and any person employed by him who contravenes a provision of section 18, 20 or 21, is guilty of an offence and is liable to a fine of $500 to $1,500 or, in the case of a second or subsequent conviction, to a fine of $1,500 to $3,000.