XenForo doesn't send out reminders letting users know if their account upgrades are due for renewal. If you use the auto-renewal option, that's not a big deal.
This little add-on makes a cron job that sends an email letting users know their upgrade will be expiring in three days and provides a link to the account upgrades page.
After install, double-check your cron schedule to make sure it runs at an appropriate time.
Couple of small issues:
- The html version of the email isn't particularly "pretty". I don't care enough to fix it up, but you might.
- XenForo won't let you renew an active upgrade. You have to wait until it expires. Nataan's "Advanced User Upgrades" will allow users to extend active upgrades. Other add-ons may offer similar functionality, but this is what I use. Alternatively, you could edit the date range in Model.php from =<3 and >2 to =<1, etc.