I go into the post oriface with whatever knife/knives/weapons I have on me at the time and dont give it another thought.
I use my knife to open packages right there in front of G_d and everyone - no one has ever said peep, only a few people give me freaked out looks. Fuk'em if they arent significantly evolved as sentient beings to recognize a good tool and its useful potentials I dont give a shtit what they think anyway.

(Not that I do anything to intimidate or otherwise freak out anyone, I am very carefrul not to, pinch my blade by the spine to smake it look smaller, dont wave draw, etc. etc. - if people still choose to be freaked out, I dont care. Would like an apology from them at being forced to share my oxygen supply, but I'll let that pass

)
(P.S. I have a lot of respect for postal employees who do their job well, and who deal with all the stress of working at the post office. At my local post office the post mistress and her one employee are not good at their jobs, or dealing with the strees [altho, really, how much can there be in a town of less than 1000 people?] and to top it all off, they are both bitches in the extreme. So dont take my post orifice comment personally, its just meant RE my local PO.)