Airtel — Bill payment after disconnection

After being a customer of Airtel broadband for more than 12 years... I requested disconnection as i had to travel abroad. Never did i skipped by bill payment. Now, Airtel calls me and ask me to pay the last bill amounting to about 500/- rupees, which is fine with me but the problem is Airtel asked me to pay this amount in cash physically in on of their offices. I can't pay it online... i have always paid my bills online however... i guess Airtel wants me to travel back only to pay the last bill... Crazy!!!