gave in last night and bought it. it took 30 min to download and 20 minutes to install. emailed apple and within 45 minutes they emailed me back saying they would refund me the $19.99.
Dear Robert,
Aaron here from the iTunes Store. I understand you purchased "OS X Mountain Lion" and then noticed that your computer was eligible for the free upgrade. When it comes to your money, I can certainly appreciate how important it is to feel that you are treated fairly, and I would be more than happy to help you out with this today.
I'm happy to report that I have been able to reverse the charge. In five to seven business days, a credit of $21.34 should be posted to the credit card that appears on the receipt for that purchase.
If you have any further questions or concerns regarding this, please let me know and I would be more than happy to address them for you. Thank you very much for being part of the iTunes Store family, Robert, and I hope you have a great day.
Sincerely,
Aaron
iTunes App Store Customer Support