Sleepypod has a new version of their harness that does not use the LATCH and is less expensive:
http://sleepypod.com/clickit-sport
My 13 lb pug/jack russell mix barely fits the minimum requirements of the smallest size. It would likely not work until the pup is mostly if not fully grown. You could start with their carrier if you have the space/budget.
I have also seen some small plastic travel carriers that have a place to route the seatbelt in the top. This holds it on the seat with the door toward the front of the car. Not crash tested, but might work for you while the pup is small, and is a cheaper option. Here is one:
http://www.petmate.com/vari-kennel-ultra-small-pet
In between is the SturdiBag, which I have. It has loops for the seatbelt on both long sides. In my car, I have the bag facing the side door and route the seatbelt through the loops on the outside, not against the seatback. It has various sizes and is cheaper than the Sleepypod. I trust my installation of it more than the plastic carrier option, too. I have a large SturdiBag, but would suggest a smaller size for the puppy if you want to move to a harness later.
http://sturdiproducts.com/pages/pet-carriers