I knew of someone who sent some pre-filled syringes to a friend through the mail in country. When they received the package it had tape on it and the pre-filled syringes were gone. The friend's guess, considering that the package had two syringes side by side was that the X-ray, seeing two metallic wire like objects side by side (the needles) it thought it was looking at something else entirely but that when they opened it they realized it was not something dangerous but they nonetheless seized the pre-filled syringes and that was that, no letter, nothing.
I would rest assured, this is a scam, the real post office is not going to sweat you over something this small and certainly not by email