Imagine if you don't have a criminal record, nor can the FBI gather evidence that you are a local drug dealer. You have you apartment cleaned up, and your tails with all DNM activity is encrypted. Now you accept a CD and open it within the next few mins and the popo executes a search warrant, within the next 30 mins.
How strong of a case does the prosecutor have on you? Also would it make a difference if you kept the pack unopened for a week?
In general, it seems like they use the CD as probable cause to search your house. In the reports here, it seems like they usually charge people with the stuff they found in the house, and not for the package itself because it is hard to prove you ordered it.
Theoretically, if you have a clean house you would be safe because they can't charge you just for the package. I haven't read a story that contradicts this theory, but I could certainly be missing an example.
It does seem like 99℅ of the CDs are from people talking, not from them discovering a package out of the blue.