My legal comprehension skills may not be up to par, but the wording of the PACT ACT does not exclusively prohibit the USPS from shipping cigarettes and smokeless tobacco. Maybe I'm reading the wrong version. I've looked at several different versions and that particular "non-mailable" section discussing the USPS still basically says the same thing.
In summary it says the business must comply with all new laws including the new shipping label instructions and must have already paid all excise taxes before shipping the product. Also, the recipient must be a verified adult of 18+, and the business can't be on the "non-mailable list" due to previous infractions, and of course it has the new anti-terrorist references in it. So... if you're not on the "non-mailable list", and you have authorization to ship, then you can still use the USPS.
All they want to do is cut down on smuggling, make sure all taxes are paid, avoid sale to minors, and make sure the paperwork is a real pain in the ....... Sounds like typical government redtape to me.
There's other things in it I really don't like, such as... a limit to how much you can mail in weight (10 ounces, if it's RYO), a limit on how many cigarettes can be mailed (12 packs), and the consumer can't order more than that within a 30 day period from the same supplier (which means if you want 24 packs, then you have to order them from 2 different suppliers).
I guess e-liquid would be considered as "smokeless tobacco" too [in the future], but it's not specifically mentioned in the Act. How would you convert 10 ounces of tobacco into 10 ounces of e-liquid when the 2 are nothing like the same anyway?