Chris,
It should be possible to tie a subscription request in with eCart (and by extension Powerstore) with some hand coding. PayPal will then automatically renew the subscription without any need for intervention unless either party cancels it. I have several clients who pay me this way for yearly services.
If you want an email sent from the site as well that would need some kind of trigger to make it happen, either a cron job or similar or someone going into the admin and clicking a button to run a check on the database for accounts set to expire in a window of x amount of days.
If you would to explore this further please feel free to contact me via my website.
