No public utility shall make or give any undue or unreasonable preference or advantage to any particular person, company, firm, corporation, limited liability company, or locality, or to any particular character of traffic or service in any respect whatsoever, nor subject any particular person, firm, corporation, limited liability company, company, or locality, or any particular character of traffic or service to any undue or unreasonable prejudice or disadvantage in any respect. No public utility corporation, directly or indirectly, by any special rate, rebate, drawback, or other device or method, shall charge, demand, collect, or receive from any person, firm, corporation, or limited liability company a greater or less compensation for any service rendered or to be rendered than it charges, demands, collects, or receives from any other person, firm, corporation, or limited liability company for doing a like and contemporaneous service under the same or substantially similar circumstances and conditions. Nothing in this chapter shall prohibit a public utility from entering into any reasonable agreement with its customers, consumers, or employees or from providing for a sliding scale of charges, unless the same is prohibited by the terms of the franchise or permit under which such public utility is operated. No such agreement or sliding scale shall be lawful unless and until the same shall be filed with and approved by the commission.