The purchaser of a used motor vehicle shall pay, in addition to any fee required under Chapter 501 for the transfer of title, a transfer fee of $2.50 for the transfer of the registration of the motor vehicle. The county assessor-collector may retain as commission for services provided under this subchapter half of each transfer fee collected. The portion of each transfer fee not retained by the county assessor-collector shall be deposited to the credit of the Texas Department of Motor Vehicles fund.