Spain Uk24

Hello..I have received the form showing which missing years I can pay for. I have 18 years of which 1year is class 3 and one year has two different amounts which I presume are class 2 and class 3!! Do I pay one lump sum for all the years or do I pay separately. I live in Spain and will be paying from a euro account when I pay is it automatically exchanged into Euros or do I have to send extra incase exchange rate changes. What happens if i haven't sent enough due to exchange rates or too many Euros??..