I agree completely- these companies have already been cut off- it is not an attempt to get them to give us more work- it is only an attempt to recover the fees they have stolen. 
My initial thinking was that given an option to pay the amount due "now" or be charged late fees of "X"-maybe they might pay up- even if there is no likelihood of ever doing business again. I recently send out bill to a company that is not going to pay anyway- they have folded- but it was kind of fun to have a charge of $50 and a late fee of $303 ( 303 days late)- I know they won't pay the late fee- but it drives home the fact that they are delinquent.
Does it create any legal problems by just including a "late fee-option" on the original billing and or invoice even if there is no agreement between the parties.
Just as you, I have never charged a late fee( seriously) or ever expected anyone to pay it either- just was the incentive factor to pay up or to point out the late payment policy of their company.
Thanks to both of you for the input.

Steve Meinecke