Ever have trouble getting paid?

Normally I will get a break down in payment. If its a company I dun trust, 50% upfront, 25% on submission, 25% on final. It sounds like they are planning to rip you off on this one. Do you have a written agreement with them? If you're looking for freelance work, try getafreelancer.com or rentacoder.com (no, I dun work there:p) There is always folks looking for writers there.

Oh the joys of self-employment. Unfortunately I know from bitter experience that there are many unscrupulous companies that know all the legal ropes and know that there is little you can do to make them pay up.

A few years ago I had to take a company to court to get the $40,000 they owed. They paid on the steps of the court house the day of the trial. Yes, I got the money, after 18 months and $9,000 in solicitor's fees.

Sorry to go on about it but this is something that really stirs me up. Now to your problem:
Did you stipulate your trading terms on your invoice?
Did you get the company to sign a contract with the agreed amount to be paid and when payment was due?
(Both these things help if you need to take legal action).
Otherwise, the squeaky wheel gets the oil - pester them daily for your money - phone, email, re-invoice, turn up in person. If that fails check if in your state there is something like a Small Claims Tribunal (which is inexpensive for people owed money to claim it - your local court would know - or check the Internet).

Good luck.