Airtel — Need my last month recharge bill

Today I selected option in airtel thanx app for prepaid to post paid. Without any prenotification company immediately convert in post paid. Now I want to download my last month recharge slip but customer care executive clearly said "We can't do that because there is no history of prepaid now". I simply asked about my last month recharge slip in mail.

I already sent a mail for this issue but didn't get any solution in that. I am totally frustrated with airtel and now when I asked to discontinue all services with airtel immediately but executive inform me "You can't do this till 90 days". What is going on? .

I want solution immediately.