Your boss is absolutely allowed to open an envelope on your desk if it's clearly a business-related piece of mail, and your employee handbook almost certainly says that your corporate Slack account is only to be used for business purposes.

In fact, your boss is allowed to open any mail sent to your work address.

https://www.azcentral.com/story/money/business/abg/2015/07/1...

http://www.askamanager.org/2010/08/is-it-legal-for-my-boss-t...

http://employment.findlaw.com/workplace-privacy/privacy-at-w...