One of the main clues in Mr. Monk and the Sleeping Suspect involves an unusually tied knot in a string tied around a package mailed to the murder victim. In reality the U.S. Postal Service has not accepted packages secured with string since at least the 1980's because the strings cannot pass through mail sorting equipment. Packages must be secured with tape.
The captain states that the bomb was mailed "three days ago." Even if the package had been mailed Priority Mail Express (overnight-2 day delivery), part of the plot was that the package had to be forwarded. This would have added 7-10 postal business days to the delivery time no matter the mail class. There is no way a package mailed "three days ago" would have been forwarded yet.