Cardira
Cardira is a CRM for service businesses. It keeps your contacts, properties, jobs, quotes, invoices and scheduling in one place, and gives your customers a portal where they can see their appointments, review quotes and pay invoices.
Google Calendar
When a business or a customer connects Google Calendar, Cardira reads and writes only the calendar events for that account. It uses them to mirror appointments into the connected calendar and to read busy times so booking shows only open slots. Cardira uses that data for nothing else.