NIC record differs from online to HMRC letter showing shortfall

I have been living abroad for several years and enquired about my NIC history Have recently received a letter from HMRC stating different amounts to those online. which are correct. also how do i show which year i want to pay, is it assumed on the payment amount and if so do i pay one year at a time?

Hi,
It appears that our International Team have quoted shortfalls of voluntary National Insurance contributions at the class 2 rate.  
Personal Tax Accounts always show any shortfalls at the voluntary class 3 rate.  
Please use the rates as detailed on our letter if you would like to pay any voluntary contributions.  
You may pay voluntary contributions either one tax year at a time or multiples.  
Thank you.