After your client signs, they will be taken to the payment tab.
From here your client can click 'Edit Amount' and adjust it to however much they'd like to tip.

If your client does add a tip, the contract will appear as "Overpaid".

To balance this overpayment and track the total amount of tips you received in a particular time frame, you can add a Service to your project named "Gratuity".

First, edit the signed contract.

Next, add the Service to this project and set the amount to match the amount overpaid.

(Need to add services to your inventory? Click here)

Finally, once the project is marked as signed (Learn more here), the project will now show "Paid".