John
I have been watching this thread with interest and offer this advice.
I own a business that offers a service to the real estate industry. We mainly deal with amounts under $1000 per customer.
From the beginning I was aware that some customers will either, not pay or be a 'difficult payer' when finalising their invoice.
Two choices for you.
Let it go and move on 
or
take legal action 
. I found that getting bogged down chasing up money that may or may not turn up was draining on all my resources.
We here in Australia have a legal avenue called - Small Claims Tribunal. For around $60.00 you can serve a writ of demand upon your customer almost immediately. He/she then has to respond within a set amount of time. If no reply is received or a resolution be arranged, it goes to court. No lawyers can be involved and it is dealt with by a magistrate. Dispute is dealt with within a short period of time, a decision is made and therefore, problem solved.
It sounds like you will have an extremely good case in relation to this matter.
Do something now and stop procrastinating.
