Gave a warning but received a ticket by mail?

Hi, We got a new car and didnt have a license plate yet but were using the toll-free road since we have the speedp. Once we were pulled up by a cop and told that the light on the speedp reader didnt go off and when i showed him that i have the transponder in the car, he said he will give me a warning and that i have to display it. I said ok and promptly put it up on the shield. 2 weeks later i receive a ticket for $177 (traffic school fee) or $155 (bail fee). Can i contest that ? can he do that - give warning verbal and then send a ticket later?