Intresting, normally it isnt a problem because when you tell the printer what paper size to use it adjusts for it. My program simply sets the paper size and then the X & y cords to where it should print. Using the paper size as the area it is allow to printer to.
On my printer if I tell it to print from the normal paper tray then it prints starting at the left, but when I tell it to print from the manual tray it pulls in through the middle, and the print comes out fine.
So yours is pulling from the center, just like my printer does, but for some reason it sounds like it is treating it as it is a full sized paper instead of an envelope. When you choose the envelope size in the program do you choose one of the printer supported ones or one of the custom ones?
Also I just put out v1.5.2 of the program, so make sure to try the new version as well
Shane