Punchbowl does offer an Automatic Reminder to be sent at a time of your choosing. Reminders are off by default and you can set one to be sent from the Invitation Options page.
You can set a reminder email to be sent up to 10 days before your event date. You can also designate if you want the reminder to be sent to guests that have responded, guests that haven't responded, or both. If you need to send a "manual" reminder message, just go to your Manage Invitation page and use the "Email Guests" tab.