Interesting about the dealership handing out the extenders. When I was reading about seatbelt extenders a few months ago, most of the vehicle manufacturers seemed dead-set against using them for carseat installation (fear of liability issues). They would only knowingly provide them to adults whose girth was too large (to put it politely) to fit in the standard seatbelt. As I recall, some companies (e.g. Toyota) even had the policy that the individual in question had to appear in person at a dealership to be 'fitted' with the extender. In other words, you can't just claim to have a passenger in your vehicle who is too large for the seatbelt; that person actually has to make a personal appearance to prove that they have the problem and need an extender.
I don't know anything about whether extenders are actually safe to use in carseat installation -- I guess it would depend on where that extra buckle ends up....
Katrin