If you have silver or gold status you should be able to choose your seats at no cost in any cabin at the time of the booking, but the Iberia web site can be hit and miss. I checked my wife in for a BA flight operated by Ibetia today and was transferred as normal to the IB system, where I was offered a Poug for 60 euros. I accepted this but then as per usual the web site crashed. Upon logging back in not only did I not have the Poug but the system had moved my wife from a prebooked legroom seat to another seat further down the aircraft. Any 10 minute with a ver helpful gold line operative eventually sorted it out but it us a poor system.