eb2hope, having the I-485 receipt is for a different reason. You are supposed to be inside the US on the date of filing the I-485. Until you see the receipt, you don't know for sure when they considered it to be filed. It is not really about actually having the receipt in one's possession; it is about knowing what date is on the receipt.
So if you do something like send it in on the 10th, travel on the 15th, then come back on the 30th and see the receipt date as the 16th, USCIS may deny the I-485 because you were out of the country on what they think is the filing date (the 16th).
Of course, you actually were in the country when USCIS received it and you have the Fedex tracking receipt to prove it, and you probably will eventually win the appeal or motion to reopen because of that proof, but that's going to cost you $$$ in lawyer fees and several extra months of waiting (or years if retrogression sets in while you appeal or reopen).
What some people do is see if the check has been cashed. Once it has been cashed, they figure the USCIS must have punched in the I-485 application into the system and already deemed it as having been filed. Often the canceled check will have the I-485 receipt number on it, so they can use that to check the status online.