Client has ignored my payment requests for the first two (watermarked) drafts.

I'm an audio engineer. The client is a real-estate company I recorded a webinar for. To protect myself, I normally have new clients pay for the recording and first round of editing immediately after recording. I don't start editing till they pay - this has happened a few times before. If I get burnt, I only lose 2-3 hours of my time recording (instead of 6-10 hours with editing). I can usually tell when this is going to happen if the client has a dis-associative attitude post-recording and ignores request for payment.

While this client doesn't give me this vibe, she does give me "corporate-pass-the-buck" vibe. Different interaction, same result: "I have your updated invoice and will process payment as soon as we get to the final version so you have an accurate estimate of your time."

I broke my own rule because this client needed a 2-day turn around. Although she's had two days to pay, each day she's given me a different excuse until today's "I'll pay you after I get the final draft". Admittedly, I've mostly worked with individuals, not companies, and now I'm worried I'm too far into this and will lose the 10 hours I've already invested.

How would you handle this? Send another watermarked draft? Stick to my guns? Is there another alternative this late into the job?

Thanks in advance!