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.
This is what we did, and we got an extension just two days after submitting