Pricing starts at $19 per month. You can install the app for free. However, you need a license key from https://smsreminder.app to set the reminders. 

SMS Reminder for Google Calendar™ allows you to schedule SMS reminders for your customers' appointments. You won't need your staff to call and remind each person. Start reducing the no-shows now!

It's very simple to use. You only need to introduce the phone number of your customer and the date and time of the reminder. That's it! your client will receive an SMS at the scheduled time.

Give it a try and start reducing the no-shows!