I was in same boat last year. I did use paper though, and included both of my parents in one application. You need to pay biometric fee for each of your parents or applicant.

You can calculate the cost using this page https://www.uscis.gov/feecalculator

Regarding timeline, last year it took way long than I anticipated. They took one month to get the appointment letter for the fingerprinting, and almost 8 months for RFE. My Parents appeared for the biometrics, but by the time I got the RFE letter, they were already in Nepal. So, I had to reply the RFE stating that they have already left the country and no action is needed. They did send me the letter saying that their VISA was extended till the date they left US.

RFE regarding some documentation to show that they will return to Nepal on time.

Hope this helps.

@Nagios

Thank you so much for your kindness in sharing your experience. Thanks a lot for the calculator link.

Regarding the timeline, I think its going to be quite like yours, where my parents will probably be gone by the time approval arrives.

From what I have understood, once your I539 application is received your stay is not deemed illegal until approved or denied

Scenario 2:

My parents visa is expiring in September 2022

I just came to know that once a B2 visa extension I-539 is filed, When your parents want to renew the visa they will have to sit for an interview at the US embassy in Nepal again ,instead of just getting an interview waiver for the the renewal. They will have to explain the circumstances under which the extension was filed.

and if their explanations are deemed unsatisfactory by the officer, the renewal of visa could be denied.

Do you or anyone else in this forum has any experience to share about this re-interview scenario after I-539 application for B2 extension?

I am in the same boat..l am planning to do paper-based just due to the cost