I just paid a non-Airtel broadband bill with no usage fee using an older version of the app I had installed on an android simulator on my PC. Just before this, I tried paying the same bill on my phone (iOS, latest version app) and it showed a usage fee.
This is ridiculous that the fee is based on the App version. That is so disorganised. And for everyone - if you want to escape the charges, disable updates on the Airtel Thanks app until they catch up with this (which can't be too long).
Edit: the fun bit with this thing is, you don't even know whom to complain to. Axis will say it's Airtel that's levying the fees. Airtel customer care will say Airtel Thanks app is not under their jurisdiction, contact Airtel Payments Bank. Airtel Payments Bank will say they have nothing to do with the Thanks app, contact Airtel mobile for it. I've been stuck in this loop before, even with escalations. A complaint with the RBI banking ombudsman quickly resolved issues.