We used to use HazmatU, but found it was lacking in many ways. First off it was fairly long and boring, and I had no way of asking any questions.
Someone I know recommended Safety Regulations. They actually do a live interactive webinar every month. You can ask questions and the instructor walks you thru the shipping papers. The class was one of the shortest at roughly 3 hours and they went thru shipping papers with real life examples.
It includes 3 years of phone support which helps me out tremendously because I tend to forget the little details.
Here is the link to sign-up for their hazmat training: