I-751寄出over 3weeks. No 收據. 臨時綠卡expire soon. Help!

I sent out the I-751 petition to CA service Center on April 2. I included the check $275 in the package and saw the check was cashed on April 6 or April 9. (From the image of the check copy, someone wrote 4/6/07 on the front of the check with some other number, but back of the check has the date 4/9/07). Right now, I still haven't receive any receipt from USCIS. My conditional green card will be expired on May 4. What should I do now if I still haven't receive the receipt? I want to get some kind of number or reference from USCIS before my Conditional Green Card expire (in case my I-751 application has any problem). Please advice is there any number I can call or so I need to make an Infopass appointment? (Even I make an infopass appt, it may pass May 4?). Please advice! Thanks!!