I-485 sent on 09/20 via USPS express mail overnight
USCIS received it on 09/21
Check cashed on 09/26
However I noticec on the back of check, the time stamp is 09/22. So it means TSC processed the package just one day after they received it. So if you want to catch the 09/30 deadline, I will suggest you send out your package by tomorrow.