Tex. Rev. Civ. Stat. art. 4013

No corporation, company or person mentioned in the first article of this title shall directly or indirectly, by any special rate, rebate, drawback, or other device, demand, exchange, collect or receive from any person, firm, association or corporation a greater or less or different compensation for any service rendered or to be rendered, in the transportation of passengers, properties or messages, than it or he charges, demands, collects or receives from any other corporation, person, firm or association of persons doing business in this State for a like service under substantially similar circumstances and conditions except as is provided in this title, nor shall grant any free transportation or franking privilege to any corporation or person except as provided in this title.