In some situations you may not want to charge a client for a shift. If this is the case, you can create a price that rounds off to $0 per hour or $0 per KM.
To create the price, from the price page click on the blue Actions button and then select Add Price Book.
Complete the price setup with $0.001 in the Per Hour and/or Per Km fields. Don't forget to create a rate that covers each of days of the week.
The shift will then be excluded from invoices as it results in a $0 charge.