I had applied for my post-completion OPT on March 20th, 2018. I was due to graduate on June 17, 2018 and I was going to fly out the very next day back home with my parents (this was booked waaay in advance, and in hindsight, should never have been done). For this reason, I wanted to apply at the very earliest. March 20th was exactly 90 days before my graduation date and the earliest date at which I could apply, hence I sent the mail to USCIS on March 19th by priority mail.
Within a few days I received the receipt notice from the USCIS by regular mail, with my receipt number with which I could track my case’s status on their website. Little did I know that this would become my most hotly visited website EVER.