Truthfully, Sparteye, I've been using both, interchangeably, all my life, and I've always wondered the same, but never bothered to LIU. I think I've come to use en- more frequently, though. The only rhyme or reason for the use of one or the other I can recognize offhand, in my personal lexicon, is that I believe I'm more prone to use ahn- in a more pointedly formal situation like asking someone at a wedding if they presented the couple with their ahnvelope, yet; or for an ahnvelope to be presented as an award (or one I ask for in public or onstage to open to read a listing), etc. And I usually use the ahn- alone...when it's in combination it's en- as in "business envelope" or "self-addressed stamped envelope". (now I'm wondering why I do this).

In the verb form, of course, it's always en- with me. "We can envelope the whole area." But, then, the suffix changes here, too, from -lope to -lup [en-VEL-up]