Police arrested three men accused of selling thousands of pills of meth-laced “Adderall” on various darknet marketplaces and mailing them through the United States Postal Service through a fictitious business called “Professional Paper Filing Inc.” that listed a real return address of an uninvolved business. That business then told police that it was repeatedly getting packages of pills in the mail as “return to sender.”
The men face a maximum possible penalty of life imprisonment.
https://www.uspis.gov/wp-content/uploads/2019/05/USPIS-FAQs.pdf
Per #4 they need a warrant to open first class mail only. All other mail they can open without a warrant.
If it’s first class and they think the contents violate federal law then they can get a warrant. I doubt many legitimate pharmacies etc. ship pills via first class, and certainly not with a bogus return address. If they saw a pattern of that I would expect a warrant wouldn’t be too difficult to get.