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.

Hello HMRC Admin 20 I too have a letter that says the amount of voluntary payments differ, considerably, to the amount stated online. Do I pay the letters amount of GBP 163.80 rather than the GBP 800.00 (approx) that it says online. Also, why are they different, it's confusing and unhelpful? Thank you very much.

Hi Annep313,
I can advise that the rates shown online are automatically quoted at the voluntary class 3 rate. 
When an application to pay voluntary National Insurance contributions ( NICs) for a period abroad is received, the customers circumstances are reviewed and
consideration is given to whether the conditions to be eligible to pay at the cheaper voluntary Class 2 rate are met (voluntary Class 2 NICs can only be paid for periods
abroad if certain conditions are satisfied).
It appears that it has been determined that you meet the voluntary Class 2 conditions and the letter you have received details to voluntary Class 2 NICs you can pay.
You should therefore treat the amounts quoted in the letter as correct and follow the instructions on how to pay should you wish to make payment.
Thank you.