I receive bunch of scam letters in to my mailbox. One looked like official gov printed notification and started with words "You might be subject for 5 years in prison" and then smaller letters "if you prevent delivering this mail". And then actual ads - extend warranty of your car..

Same goes about "won tickets to cruise" with phone number where they offer you to buy something...

So in this regard USPS is not better then email. It is just happened that sending bio hazardous stuff is expensive through snail mail..