Based on the position you are in now, I dont think you have much of a choice other than to provide that letter as requested. You are right that union positions usually have a set grid for each position, my experience is that new hires start at the bottom. You may be able to request a review through the union that takes into account your years of experience and allows you to move up the grid after being hired. I know this is possible in some unions. If you are still interested in the job and have already resigned from your current job you should just provide the letter and not worry too much about it. Not much that can be done now anyway! And you will know for next time.