Pay ‘em online.
Prints out a timestamped receipt the minute the payment is accepted, and the actual electronic cash transfer could take as long as three days (If you make the payment late on a Friday and like driving on the stripes).
Only downsides are: Deleting all the spam email you get from them and the probable grade A pain in the butt if Citi goes tentoes and you have to reconfigure to a new system.
I have a Spam filter..:)
sw