Cal 1-800-FOR-MERC and ask to speak to someone in the high mileage awards department. I have gone through it many times with my high mileage cars. They'll send you an application, fill it out (in the meantime, get your VIN # and engine # handy) and mail it in. They will send you a badge and a certificate within about a month. The awards are at 250,000Km (155,500 miles), 500,000Km (312,500 miles), and 1,000,000Km (615,000 miles). Hope this helps!
'67 250SE Cabriolet
'93 300E 2.8