To wrap something up in a covering is to envelop it (pronounced “enVELLup” ). The specific wrapping you put around a letter is an envelope (pronounced variously, but with the accent on the first syllable).
First-class ticket: more legroom, more emissions First-class air passengers have a carbon footprint as much as seven times larger than the average passenger's, according to a new study from the World Bank. The study comes as airliners push the envelop of luxurious accommodations, while simultaneously aiming to 'green' their operations.
June 17, 2013 - The Christian Science Monitor
National Envelope Hopes To Lick Bankruptcy Filing National Envelope, the largest privately-held manufacturer of envelopes in the U.S., has filed for bankruptcy protection. It's a sign of the paperless, digital times. It previously filed for Chapter 11 protection in 2010.
June 11, 2013 - NPR