Well several companies here charge late fees and it is just a part of the package you give your clients.  We have packages to fill out and many abstracting companies have packages to fill out.  If we have certain terms and conditions, then so should they.  The reason you would want to do this is perhaps a resistance to sign it in the beginning with a company could give you a clue of what to expect would be one reason.  Clients that pay will not have a problem with signing.  The other reason is, if you sue in small claims court the late fees are tacked on to the total amount of the judgment along with post-judgment interest at an amount usually regulated by your state.  One abstracting company here takes credit card numbers too and if the invoices get too far behind, she has an signed authorization to charge it.