Here is the info on the site.
18x8 inch 18x10 inch, 5x130mm 50/47mm
Silver Wheels
It looks like the offset is 47mm for the rear. This place says they will fit the boxster so all should be fine. 47mm sounds good to me as I would like the wheels to be pushed out and more flush with the fenders.
http://www.lakeshorewheelandtire.com/specials/porsche-boxster.shtml#18inch