Dan,

You shouldn't be apologizing to them, they have owed you money for over a year and are just now paying? And the reason they treat you like crap is because they KNOW they are wrong. I am sure you turned their order(s) around quickly and then a year later are being nasty, you should merrily say goodbye to them.

I always say to the person, would you still be working there if you hadn't gotten a pay check in a year (3 months or however long its been that they owed me the money) and I say of course not. So why should I be waiting?

Last year I got on the phone with a very small client I had, there was a pattern of them going out 90-120 days with me, and I got fed up with it (after many many attempts to politely handle it), time is money after all. At the end of the phone call the guy said "it was a pleasure doing business with you", and my response was I wish I could say the same!

I was very happy the relationship was severed.

In the long run it ends up costing you and I more money  having to chase up a few hundred dollars here and there, its just not worth it.

Fortunately now I have clients who just pay me. Once in a while invoices get missed and my book keeper emails the invoices to them and they get paid. This is as it should be for all of us.