Billing Reminders for short term schedules (EZ schedules, One-Day Schedules, or Pro Schedules) appear in your request queue on days that short term schedules end (labeled "END Schedule"), as well as a number of days before short term schedules are to begin (labeled "Start Schedule"). These are sent to remind you to bill clients before service begins or as soon as service ends.
Billing Reminders are sent only if the "Send Billing Reminders" box is checked on a short term schedule. If the box is not checked, then you should not expect a reminder.
You can make sure that the "Send Billing Reminders" box is checked by default when you create a short term schedule by setting the "Send Billing Reminders" preference to "yes" (in ADMIN > Preferences > Billing Preferences).
Please see the How-To document How to Use Petsit Billing Reminders for more details.