Just looked at that site, and although I can't help with your exact question, could you advise what car you are buying wheels for?
Your sig indicates you have a 2007 (C6), but you are asking this in the C5 section.
All the Cray wheels listed on that site are offset to best fit a C5, not so sure they will fit a C6 quite as well (rears
may stick out of the fenders a little).