If you want to get into an already-sealed envelope without ripping it apart, all you need to do is pop it in the freezer for an hour or two.
Whether you forgot to include something with your letter or you have more nefarious schemes, this method should open up an envelope as if it were never sealed shut -- with less mess than the more popular steaming method. Just throw it in the freezer for an hour or two and the envelope should open right up, with no one the wiser. Hit the link for more clever freezer uses.
8 Weird, Non-Food Uses for Your Freezer [The Secret Yumiverse]