I just checked and yes they say the envelope cannot be opened. So, it would more or less be open at your own risk if you were to try.That's why I asked it didn't seem as if it was able to do that and to avoid damage I didn't attempt it.
Page 04 under "Cautions 02", line item 9-"The wax seals on envelopes are made of ABS plastic and cannot be opened"
Instructions are there for a reason