you're missing the other piece of this. yes, USPS will seize mail if they discover there are drugs inside and get a warrant. however, if they do that, they can't do shit to you if you don't then go and try and claim the mail from them. they literally can't prove you mailed it or that the recipient requested it because they don't verify identities when accepting packages. this is why virtually all sales of grey or black market substances goes through USPS -- there are strong rules about what they can and cannot do and you lose those protections with the private companies. it's literally a felony for USPS to discover you have drugs by opening your mail.

If you check sellers on a certain platform, they all ship through USPS for a reason. It's safer overall and requires less identification.