You have to be careful about assessing late charges. The law permitting them is state specific. In Connecticut you can not charge interest/ late fees unless there is a statute that permits it or you have a contract with the client that permits it.

You also have to be mindful of usuary laws in the forum state. Although I think that a commercial transaction such as you describe is not subject to these laws, it would be a good idea to consult the law of the forum state. In Connecticut usurious interest/late charges make both the principal and the late charge uncollectible.

In terms of collection...don't waste your time with collection agencies. All they can do is make threatening telephone calls which are limited by the Fair Credit Collection Act and write annoying letters. You are better off serving the client with a summons. Most of the time it is enough to get you paid, and you do not have to go to the expense of litigation. Sometimes just sending them a copy of the summons with a cover letter explaining that you will serve it if you are not paid is enough, but be prepared to follow through with serving it. There is something impressive about a marshal appearing in their place of business, flashing a badge and serving the papers.

Publishing your complaint on the internet or other media is not a good idea. It could lead to an ugly and expensive defamation or trade libel suit. While the truth of the statement is a defense, you would still have to go through the expense and aggtavation of defending yourself. By contrast the same statements that may be defamatory are covered in most states by a qualified/conditional privilege when alleged in a civil action. Statements made in court pleadings, depositions and testimony do not give rise to suits for defemation. However, the key word is qualified/conditional privilege. The privilege can be lost by over publication in the media. We had an ugly divorce case here in Connecticut a few years ago which apparently gained a high profile in the media. One of the litigants chose to publish remarks in the media while the case was pending, and lost the privilege through over publication.