That's exactly what I did. Wrote a letter explaining that I submitted the application on August 1st with proof of delivery obtained from the USPS showing actual signature of person who signed for it, time and address. I also mentioned that with my deadline being the 7th that it was highly unlikely that I would receive the PCC before then and that I will submit as soon as I received it. In addition, I added the receipt for the fingerprint and the mailing of the documents. I did not have a receipt for the money order, but if you have that, add it too. Lastly, I included an email communication between me an FBI showing CIC that I requested the status on my application and FBI responding that they are still processing July 31st packages.
I submitted my eAPR last night so I can't tell you whether or not I got an extension yet but it seems like that's what most everyone else did when they had yet to receive the PCC from the FBI. Hope that helps.