Last moment Visitor visa extension

Guys need quick advice for my situation:

In law i94 expires on: 11 December

extension forms sent thru fedex overnight mail: 29th Nov (yesterday)

And her actual return flight : 4th Dec

Q) can she stay without receiving receipt notice. (Incase receipt comes after her i94 expiration) ?

It is a good idea to notify CBP office at airport about the extension request but not before December 7th. Take all documents to show. However if there is no CBP office nearby, you have your Fedex tracking evidence and in any case the file will show as “received” by Monday.