Thanks for your response.
I have moved to another sponsor in June 2017, for 4 years visa (expires in June 2021). My family had previous visa until 10 Oct 2018 so I didn't apply for them at that time.
It's been over 1 year now since I got my 4 year visa, now only 2.9 years left. I have now applied to extend my family leave by further 3 years (2.9 yrs) and paid fee that covers 3 year visa. however, I received letter to pay visa fee that covers more than 3 years
They said -- If I don't pay, my family visa will be rejected and they will become overstayers as they leave expires in few days and any new application will be considered as if it's applied after leave is expired.
I have sent my card details as I cannot take risk at this moment but not sure if I can appeal this and get the fee refunded? Any help will be greatly appreciated.