Threaten a small claims court. Do not delay.
Not a commission sale but, when I was self-employed I had a customer go bust on me. I gave them way too many chances, culminating with a cheque that bounced (it was some time ago), and after that a letter from the reciever.
Second example was a commision sale of an old Leica camera where the bloke (Peter Walnes - be warned, he's a scum bag) sold the camera, gave us excuses, and then went bust owing us around £750.
Don't delay, go straight for the Jugular.