You should have received the receipt by now. Mine came in about 7-8 business days after I mailed my documents to USCIS. I applied at the end of January though. And I received my OPT card near the end of March. It's taking longer probalby because they are getting too many applications these days. You should get the receipt pretty soon. Contacting USCIS may not be a good idea as they get ticked off for no reason (One of the staffs at my school's international services told me that USCIS can deny your application for minor reasons like going outside the area designated to write/sign.)
I'd say, there's no reason to be alarmed for a few more days; you'll probably get the receipt by next week. Good luck.