Be glad it turned out the way it did...if you ever threated me like that, I would have just laughed...How many of us have bad debts? How many of us end up in a court room or jail? very few, if any, at least not over civil debt issues anyway...sure, you can take him to civil court, and even win..but a win means very little if the other side is not willing to pay...its unlikely that a collection agency would even take it, collection agencies work in heavy mass volumes...not one silly collection case...If you do not believe me, ask all of those that owe 1000's to their credit cards that can not seem to get their clients to pay up.... and even if you got a collection on his a$$, so what, they cant force him to pay either... Be thank-full it turned out the way it did...he could have reported you for making threats over the phone, and stalking...this is why we have a legal process...you decided to take matters into your own hands, so be glad it turned out the way it did...
Glad that everything turned out well. I think to properly handle this, all you can do is to be polite and send multiple reminders to the tune of "I'm sure you are busy and may have overlooked the bills, but our records show that you owe us $xxx amount of money." Keep sending reminders every couple of weeks or so via email, phone or hard mail (or whenever you have some spare time). That way, you appear professional, and you don't run the risk of getting accused of threatening, harassing , stalking, etc. If you get payment, you still stand a chance to keep him as a client. He may even refer more business to you because you are always well behaved. Now that he is threatened by you, he may not buy from you again. Yes, he may have paid up, but it's likely that he did so grudgingly, which is never good for business. In the event that he still doesn't pay up after multiple reminders, then just lose this client. To be honest, $150 is a very small amount. As others have already suggested, let it drop and move on. Everyone is better off this way.