Did you simply mail your i751 package or did you ship it via UPS or Fedex? If you can track your package to see when it was delivered that would be great.
Keep checking to see if your check was cashed or card was charged, when you see USCIS has received payment you will know your application was accepted.
It is taking three to six weeks for the lockbox facility to process an i751 application. Some recent applicants have waited up to 2 months to receive their receipt notice from USCIS.