If you need to save collection as csv, you should probably just concatenate and write it yourself.
If you want to save existing excel as csv, you'll need to change save action (or better create new action) to something like this:
wb.SaveAs(filename, fileformat, _
Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, _
True)
fileformat enum for csv is 6
True at the end makes csv ""localized"" (for example Russian excel uses comma as decimal point, and semicolon as separator), otherwise excel will save in US locale.
You can check all other options on MSDN.