[Rick Lee]

When I've had fights with my homeowners assoc. years ago (they changed management companies several times with new phone numbers and addresses and didn't tell anyone where to send the checks), they'd take my most recent payment and apply it to "past due" bills, then tell me my most recent one was short by that much. I think if you had a contract rider or billing policy that said "All payments received will first be applied to oldest outstanding invoice," then you could probably skip the discounts.

At my current company, we require a credit card number and auth. form signed before we'll do work for someone. If they don't send in payment, we charge the credit card. If they dispute the charges, we can easily document the qork request they sent in and the work we did for them. Ironically, the fed. gov't. is probably our fastest and most reliable paying client.