This is a problem most people don't think about. All that is thought of is the perfectly envelope that is lined and hand addressed. I would suggest that you create a guide and then use a pencil to go over the lines. Then you could place the guide on the envelopes face down and transfer the pencil onto the envelope. This would make a temporary guide that you could use on the envelopes and then erase after you were finished. Of course you should experiment before doing all of the envelopes to make sure that the ink will not smear when you are using the eraser. The ink you use will make a difference. I know this greatly increases the time that you will have to spend on the envelopes, but at least it is some sort of solution.