Marc,
I renewed in November 2011. I just checked the box at the top. I had added pages about two years ago. Had quite a few pages left but it was due to expire in Oct 2012 and given the six month rule so many countries use I took advantage of a window in 11/2011 to 12/2011. I do not have a record of the details of what I paid other than did it by check for $122.72. I think I paid for expedited handling and mail just because it seemed to make sense. From the amount , which included the expedited mail, I do not think I paid the $82.00 for extra pages. I for one would write two checks as that is not that big a deal. I would put both in same envelope.
Not sure this helps much but is my experience that is fairly recent.

I got the new passport in about three weeks. The old passport came by separate mail about a week later.