[AFreeBird]

Still, it's not legal for you to open someone's mailbox to read any postcards you may find in there. I'm pretty sure that the feds would have to have a warrant to look at it when it's at the post office.

I suppose technically you're correct. However, the nature of a postcard being what it is; you can't keep the people handling it from reading it, and if say a postal worker called in sick and was replaced by someone else... who happens to work for the TLA (Three Letter Agency) of your choosing.

Well...

[Still Thinking]

Or even if the postcard contained some evidence of a crime or conspiracy, and it was the normal postal worker who happenned to catch a couple words in passing, then read the whole thing, then called the fibbies. That would no doubt be admissible.