Hi...only people who have dogs that travel alot?

hello. I am a 8 year old girl... and my mother said that i have to pay for the expensise for my dog. She is an Pomerranian and is about 2 years old. OK...well we are going on a trip and we need a seatbelt for our dog. I seem to think that seatbelts are way to expensive. Can i just tie my dog's harness and leash to the car. She sits in the back seat