Regardless of what your expectations are, it's not private. Just because you expect something to be private does not mean that it actually is.
What security agencies have done is scan and collect all envelope info (metadata) which was ruled to be not private (makes sense) but with powerful enough cameras, lights, and enhancement software that most letters sent in standard envelopes can leak their contents.
Not when you have all of congress (minus a very select very few) wagging their tails and eager to please you it's not.
https://www.forbes.com/sites/erikkain/2013/12/29/report-nsa-... https://www.theverge.com/2013/12/29/5253226/nsa-cia-fbi-lapt... https://en.wikipedia.org/wiki/Postal_interception https://books.google.com/books?id=2OZgDQAAQBAJ&pg=PA97&lpg=P... http://www.nytimes.com/2013/07/04/us/monitoring-of-snail-mai... http://www.pucl.org/from-archives/81july/mail.htm https://www.theguardian.com/commentisfree/henryporter/2010/m...