I live in a border city (Canadian side) and I find crossing the border stressful. It honestly depends on the border agent you get. When traveling to Texas the first time a few years ago, I had a hell of a time getting through and the border agent interrogated me like crazy. The next time I went I got a super nice agent and he started suggesting restaurants to visit and things to do, lol.

Have your passport and your letter for the car. And cross your fingers that you get a good agent when crossing!