What's the envelope inside mac's box?
The black piece of cloth is not an envelope, but a nicely folded display polishing cloth.
Apple mentions it in the manual:

Cleaning the MacBook Pro screen
To clean the MacBook Pro screen, first shut down
your MacBook Pro and unplug the power adapter. Then dampen the included cleaning
cloth with just water and wipe the screen. Do not spray liquid directly on the screen.

You can confirm the looks on this review. And the long cable is just an extender which you can plug into your charger.