Ah, but The Oxford English Dictionary lists the possibility of using 'envelope' - colloquially - as a verb! So, "All these letters are enveloped now, sir" would mean the letters had been put into envelopes. In such circumstances, the stress would be on the 'en' and the 'ope', just as happens with the noun form.
Having said that, I've never actually heard anyone saying such a thing. I am merely being mischievous since - though it's probably of little interest to anyone else - this is my 9,000th response here on AnswerBank.