pure 485, yes, received before 8/17.
any package with 140, have to be received by 7/31.
As to whether it is postmark date or receiving date, USCIS has made it very clear before, all cut off date is the receiving date in their mail room! (though the receipt date on your file might be a couple of days later.)
So definitely send out the day before (before the express cutoff time), using next day service. Or safer, a couple of days before that!