2003: Expire and reset permission to use at users site

Hello,
Permission Expired and Reset ..... Needed for a 2003 workbook that is to be sold on a per month subscription fee. The Excel file will be emailed to the client upon purchase.
1. Something like setting expiration for 6 weeks (to allow 2 weeks grace). This seems to be an available option in Excel.
2. But then how to reset the expiration when the user pays for the next period?
3. Payment will be via paypal.
4. Maybe I wd have to email a new password each time, If so, suggestions as how to create many unique one-time passwords?
5. Would these passwords have to be installed in the original file so the workbook recognizes them?
6. Any alternative ideas?
Help much appreciated
phstol

Hi Roy,
Thanks for your insight ....
I'll look for a secure means.... Any suggestion as to where to start?
Thanks,
Peter

Thanks Rick, will do,
Peter