This deal has ended.
Through 3/22, Buy.com has the Cables To Go iPod FM Transmitter for only $19.14 after a $16 mail-in rebate. This is the lowest price so far this year. This runs both on batteries or your car's cigarette lighter. Shipping is free; there is no sales tax (except CA, MA, MD, TN). Of note: new Google Checkout users can get an additional $10 discount for signing up.