Handicap parking violation?

So i received a parking violation a few days back on grounds of parking at a handicap spot. I do have a handicap permit however on this day I just forgot to put my handicap plaque up in clear view (it was lying on the passenger seat next to me which is also in clear view grrr...). I am going to write a letter appealing for the $300 fine i received. It would be nice to get some input on if i even have a case to present here.. Any thoughts would be much appreciated!