Voluntary NIC payments

I have received my letter with my shortfall rates on. I live and work abroad so will pay with IBACS, what I dont understand is how do I tell payments what the amounts for? I have 7 years missed, I am choosing to default (not pay) on one year as it is higher than the rest as I wills till have enough years saved when i get to retirement. But i dont see any mechanism for me when paying to say this payment is for 2017-2018 and 2018-2019? How are the shortfalls identified at payment?

Hi,
You will need to contact our National Insurance team, you can explain to the advisor which tax year/s you are intending to pay for and the advisor will note on your account which tax year/s you are paying for, so when the payment comes to us it gets allocated and posted to the correct periods.
Please find our contact details in the link below:
National Insurance: general enquiries
Thank you.