We're in the
So in theory you could jus go to the form and try all numbers between 100 and 999 and see which one would give you the biggest discount. I tried a few and it seems that there was something to this theory. Sometimes it said invalid code, but often it said something like, code not valid for this date or code not valid for this region. Hmm, what could try 900 codes without getting bored?
So I fired up my python editor and wrote a small script. This turned out to be slightly more complicated than thought the site needed a cookie and stored in the cookie some of the route information. Weird. Also it, seemed to generate the form on the fly based on I don't know what. But I had my little firefox developer toolbar, converted the post form to get, looked at the cookies I had and pasted it all in my Python script and after 20 minutes or so it actually worked. I added a sleep statement to the script to not send out all queries at once (I'm trying to get a discount here, not to annoy the server) and let the script run for another 20 minutes.
Result: It is not just V185 that gives you 10% off. V311 also gives you 10% off! Oh well, at least now I know for sure.