During the time I had to send in my offer letter and request letter (for proof of expedite request), I let them know that the job start date (also my initial OPT start date) was already past. I was also able to convince my employer to modify the offer letter accordingly that they have extended my offer with a new start date and they will not be able to do that anymore. I guess it worked for me. Infact, I got a mail from USCIS acknowledging my needs on a priority basis. Might be the reason why they sent EAD card on a priority 2-day mail. From what I gathered, people at USCIS are reasonable persons if one is able to properly present his/her requirements. =)
I am not sure whether they always do on a priority mail.