Visitor Visa (B2) Extension For Parents

[naryal]

Hi , Last week I filed I-539 to extend Visitor Visa for my Parents. My Parents I-94 is going to expire on May 24th 2015. After 1 week I got notice that the amount I paid was not correct hence they returned the documents. It has RECEIPT NUMBER and RECEIVED DATE (May 4, 2015). After that I immediately correct the cheque amount and resend the document and I am waiting for the confirmation (RECEIPT NUMBER) or notice. Since I don't know whether the document has been received or not I am wondering what should I do ? Should I go ahead and do efiling instead or is it ok to wait for the RECEIPT NUMBER notification and wait for the decision. What happen if I don't get confirmation of the application received and I-94 expired during that time itself ? Will my parent considered as over stay in that case ? Based on forum and answer it seems like once the confirmation of receipt you don't really need to worry and just wait and take action after the decision from USCIS but in my case I am worry because they has not withdraw the amount from bank and not sure how long it will take for them to decide (although based on last time it seems they decide pretty quickly within 1 week) but since I-94 will also expired soon I am confuse.
Please help me to understand
1. Can I consider the first RECEIPT NUMBER as case and my second attempt with doc correction as case in pending and wait for USCIS decision regardless of I-94 expiring date ?
2. If I don't get any RECEIPT NUMBER (or confirmation that USCIS received my application) before I-94 expired date should my Parents leave immediately or can they still wait for decision from USCIS ?