I am owed 4500 for work performed this year. When can i take this company to claims court? What is the process? Cost?

I am an independent contractor and my last invoice was send on 2/28. They have paid back approximately 2500 total but the communication about the remaining is getting few and far between. They have avoided many calls and queries and promised a few times the check is in the mail. I need to take the next step if odds are in my favor