B2 Visa Extension -Clarification

Hello sir,

My mother in law came to US on B2 visa.As per the I94 she can stay up to 05/21/202 (May 21st).
Due to COVID -19 all international flights are cancelled.She could not able to return back .
I applied for extension,Send all required document through UPS expressive delivery.
As per the tracking number document will reach USCIS office on May 18th.

Since i applied for extension little late, Just want to make sure the her stay will not move to illegal status.
So Please confirm the below Questions sir.

1.Is it Ok if my extension document will reach USCIS three days before last admit date?.
2.Are we safe once USPS package (i mean documets) will reach the USCIS mail box office before the last expiry date?,
3.When the status will change to Document Received ?.
I heard that once documents reach USCIS mail box thorugh UPS or fedex then it will consider as document received and we are safe.
Is that true? Please confirm. If not so then please let us know the actual flow.

Regards,
Ganesh.

Nothing is safe until USCIS generates a receipt number.

The ‘receipt date’ mentioned on the official USCIS receipt letter is considered final.

USCIS recommends sending extension application 45 days before expiry.