Texas Election Code - ELEC § 202.007. Filing Deadline for Application of Independent Candidate

(a) If a vacancy occurs after runoff primary election day, an independent candidate for the unexpired term must file the application for a place on the ballot not later than 5 p.m. of the 30th day after the date the vacancy occurs or 5 p.m. of the 70th day before general election day, whichever is earlier.

(b) An application filed by mail is considered to be filed at the time of its receipt by the appropriate authority.