Personally, I wouldn't start to be worried about it yet because if you mailed it on the 15th and if it takes two days to get there, you have only been waiting 8 business days for it to clear. That is not really all that long in the government world of handling anything.
How often do they return the receipt? The last few paper returns I did, they just didn't return the receipt.
You have to get an explicit return receipt form, which they paste on the envelope. When the envelope reaches the destination, the mailperson detaches it, writes down who signed it, and mails it back to you.
Another alternative are tracking numbers if you mail something certified or express. Those you can check the status of at the usps website.
I mailed both my 2007 return and my first 2008 estimated tax payment on the 15th. I was getting a little concerned with the delay in the IRS cashing the checks, but both checks made it to my bank yesterday.