i don't have the AirPrint driver installed. (at least, it doesn't say what you said it would say if it were.)
i've done an internet search for this problem with this printer and it seems that it is a VERY common problem. i'm downloading the drivers you linked (though i think i have the latest ones) so, we'll see if that makes a difference. but, right now, this thing is useless for envelopes since it can't even put the return address near the proper place!