When creating an invoice you have the option to enable tipping. This will give your clients the ability to tip you. When your client pays the invoice, they can choose between 20%, 15%, 10%, or no tip.
Start by creating an invoice within a project. If you don't know how to create an invoice, you can learn how, here.
When you're editing or creating an invoice, click "⚙️ Options" next to "Payment method".
Toggle on "Enable Tipping".
Check all your invoice items and settings. When you have confirmed that everything is correct, save your invoice. That's all there is to it, the invoice is ready to be sent.