Legal notice was issued though company terminated me

Dear sir/madam, I was working in techmahindra from feb 2017 and on jan 2018 they had terminated me from company. After that I had got mail to pay bond amount though they removed me. And I spoke to HR of company about same and told her that I don’t want service certificate since I cannot pay bond amount and she asked to mail her same and she would convey to exit team. But as time passed I got one more mail from exit team/HUB then I mailed her again that I don’t want certificate and I won’t pay the amount. But today I got notice (post) saying that we had sent you previously 2 letters and you dint reply to those (in reality I dint receive any letter or mail) and asking me to pay thfor bond amount in 15 days. And I received letter after 5 days so 10 days left and if I don’t pay then company would take legal actions. This way it is mentioned in notice. So please tell when company only terminated me should I pay the bond amount ? And when I had confirmed that I don’t want any certificate and won’t pay amount why did they send me notice ? Please someone suggest