Please please please dont buy that thing! If you read closly its for in flight use only, not even take off and landing which are the most dangerous times of the flight for a lap rider. It might help during turbulence, but thats it. Your lap is no place for a child during a crash!
Please buy your child a ticket, most babies under two get a reduced fare. take your carseat and strap you child into it. If you just cant afford the expense, bring your carseat anyway and ride flights at the least traveled times. they will let you set it up and put your child into an empty seat if the flight isnt full.