Honestly, the way I got rid of lice took a little more time but it did the job just the same--and without me having to shampoo (I was pg) or change sheets or boil or anything.
Adult lice lay eggs. Metal flea combs can comb out child and teenage lice and anything bigger than that. You'll never ever ever ever get all the eggs or nits, so I just combed my own hair (and DS1's) twice daily with the flea comb.
Depending on temperature/humidity, most teen/adult lice won't live w/o a life/food source for more than 24 hours. If there is ANY way to just avoid the car seat for 24+ hours, then everything on there is dead. Otherwise, give it a good thorough wipe down before and after riding.
Due to the life cycle of the louse, this method works b/c you are daily eliminating the only source of egg-making (as teens, before they can even make the eggs). So once the eggs in your hair turn into teens, you'll comb them out and be done, essentially.
I hope that explanation makes sense, LOL. I didn't use the shampoo because A. it doesn't work anyway, B. it doesn't work anyway, and C. chemical concerns. (on points A and B, if you'll notice the shampoos give the same combing instructions as I just stated... they just don't want you to know that it's the combing itself that works, LOL).
I wasn't "contagious" after the first few thorough combings b/c as soon as those suckers hatched I'd catch 'em with the flea comb. =P