Implications if I use advance tax head ( 100) instead of self assessment tax (300)

Hi,

This is related individual asessee  only "salary Income"

could you please guide me...........due to  this Covid -19 lock down i could not get my tax calculation details from my employer which i recently left, about the TDS deducted  for the AY 2020-21 & the Gross income calculation...

1)Hence , i did one approx calculation from my side &  felt i will have to pay  approx 40% of my total tax liability. My concern is now we already at 30th April' 2020, iam willing to pay some tax towards the income tax (lump sum), under what head i should pay it , should it be as advance tax (100) or Self assessment tax ( 300) ?what would be the implications if there is mistake while selecting the head? 

2) Any idea that there is extension of due date for advance tax payment for AY 2020-21 till 30th June '2020 ? If yes will there be any interest on  it considering it is as  delayed payment ?

Note :- why i have a doubt if i  can pay it as advance tax (100) ,  there is scrolling  showing in the website "NSDL SITE, "PURSUANT TO COVID-19 RELIEF  MEASURES ITD HAS NOW PERMITTED TAX PAYERS TO PAY ADVANCE TAX FOR 2020-21 AS WELL TILL 30TH JUNE 2020"

Thanks in advance

Siva