All the envelopes that go through the post office are copied. My guess is that if they feel you are suspicious, the government will open them, without you knowing, or getting a warrant.
Right, but you're basically referring to the NSA's unconstitutional, suspicionless, mass spying where (in transit) all envelopes are diverted, their contents from all citizens are copied, stored, resealed and sent on their way.
Of course, they can't read every piece of mail they copy, so this is all with the intention of future usage against said citizens (and politicians) who "act up" whether it be via peaceful activism, business success and/or political success. Of course, this is mostly under the guise of security theatre. Even though the NSA and some of our weaker, piece of shit politicians says that's legal; It's truly illegal (unconstitutional), against our privacy rights (see Constitution again) and that's why there's a nationwide angst building against this type of spying from our government.
On the other hand, if you deliver mail in an envelope (encrypted) via a corporation like Fedex, they can't "legally" make copies of your mail contents to read unless compelled to do so by authorities or there's a rip in the envelope and a worker spots some child porn within the letter.
The corporation can legally read where the letter goes to (metadata), but if they get caught reading the contents without any justification they are breaking the law (our privacy rights) and disgruntled employees (via adversarial journalists, etc.) can cause great financial harm from consumers as well as bringing upon the wrath of our (hypocritical) government against them for abusing our privacy rights.
In that sense, the corporations have a lot more to lose by illegally snooping on mail and it thwarts them from doing it institutionally at least. That's why we hear about where a company fires (and even prosecutes) employees that get caught spying on their customers. On the other hand, it takes a whistleblower like Snowden to ever get the government to admit that they do it at all, much less institutionally.
I'm no big fan of most big corporations, but it would be corporate suicide for them to institutionalize mass spying on their customers because without a doubt eventually disgruntled employees will come forward and the corporation can't hide behind security theatre like the government tries to do. As some corporations are finding today, even just being accused of aiding the NSA in spying is already hurting their beloved bottom lines.
Do I think there's unconstitutional corporate spying? Absolutely (they've already been busted doing it repeatedly), and it needs to be thwarted with mass encryption ASAP along with adversarial journalist watchdogs (a big problem with corporate media not doing that job for obvious, corrupt reasons). But, fortunately... there really is a right to privacy in this nation and corporate spying is at least somewhat limited by different concerns than our government has in that regard.