i would recommend brand new scraper blades and then plenty of soapy water and scrape in 1 direction only (forward) then wipe clean and repeat do not scrape forwards and backwards as you will more then likely scratch the glass. I would also HIGHLY recommend that you get the customer to sign a scratched glass waiver and explain to them although you will do your very best to remove it you cant guarantee there may be small or faint scratches left. And of course price well.
paul