The commissioners court of a county may by order authorize the tax assessor-collector to maintain a branch office in a municipality with a population of 5,000 or more, other than the county seat, and to appoint a deputy assessor-collector for the branch office. The salary of a deputy assessor-collector and the expenses of the branch office are necessary expenses of the tax assessor-collector and shall be paid as the expenses of the tax assessor-collector are paid.