On the Pay bill screen, clients will have a choice to add a tip. The tip suggestions are based on your settings to encourage better tipping practices. You can change the percentage amount to give clients several options during the checkout process.
To start accepting tips, turn on "Ask for tips during checkout" in the Checkout tab under Account Settings.
If a client wishes to tip a set dollar amount, you can add it as a custom charge on the previous screen then select "No tip".