I hope you get the receipt. Else you could try what I had suggested earlier to someone else, call up NSC 1-800-xxxx and check with them to see if they can give you your receipt# based on your A#. If they do, then you can check your case status online and take a print out,atleast something better than nothing.
The reason why I am saying this might work is because the last time I tried to efile my EAD I got an error and when I called up NSC to find out what was going on, the guy asked me for my A# and said that there are no EAD applications in the system for my A# me and asked me to resubmit.