This is the place to post questions and discussions on usage and style. The members of the Wordwizard Clubhouse will also often be able to help you to formulate that difficult letter.
Judging by the colour, I'd guess it's either a Manilla envelope (made from hemp fibres) or a kraft envelope (made from wood pulp fibres). I suspect though, you are asking about the closure. We called them "string tie envelopes". Google seems to favour "button and string envelopes".ACCESS_POST_ACTIONS