The Downside of Finding New Clients

Last week I had an extraordinary week in marketing and I must concentrate this week on reaching the sources and researching the articles for my new clients.
It was during one of these such marketing blitzes that I found another client I will be paying some attention to this week as well.
However, it's the kind of attention most of us business owners don't want to have to do - collecting on a past due invoice.
I enjoy writing the stories for this client and more important, the pay is fairly good. However, I'm working on my third issue for them and they've had problems paying me during every single one.
The first time, they were moving offices and I cut them some slack. The contract is pay within 30 days of publication and I did receive the first check within 60 days.
On the second issue, I was moving, so I made sure the editor had my new contact information immediately. I moved, went on vacation and came home - still no check. Still, although the publication officially came out at the end of May, it was the June/July issue, so I started the clock on June 1st, meaning technically, they were not past due until July 1.
I contacted them upon our return from Germany - when they were roughly 15 days late - and was assured (as I had been before) that the check was in the mail.
Having worked in the credit and collections industry for nearly 20 years, I should have known better, but at the time, I had other things to attend to - such as organizing my office and getting business to keep me in business through the fall.
Last week, however, I started the full collections process:
1). Another email detailing our contract went to both managing editors on the masthead.
This prompted another "I'll check into it" email.
2). Three days later, I sent the invoice via email once again with a bright red "past due" on it. I also printed "to avoid further collection action, please pay the amount due immediately." This is very important, by law, you cannot directly threaten to sue a debtor. I also made a phone call and left a message for the managing editor in Kansas City.
This prompted another "I'm checking into it email."
3). Today, I will call again. And my friend and writing goal buddy, Heather, suggested I get the check # this time if they say a check is in the mail.
4). On Wednesday if I don't receive the check, I will mail registered letters to both publishing offices, since both managing editors are in different cities and even I'm confused at this point who is handling payment. I will also notify writing watch lists at ASJA and through Writer's Weekly.
Unfortunately, this company's time clock starts ticking again on Wednesday, as the August/September issue, in which I have two more stories, will be out.
I have another assignment, as well as a contract sitting here waiting to be signed for the following issue, but I think I'll pass. First time, shame on them, next time shame on me. And the pay is only good if you don't have to invest additional time and money in collecting what's due.