We have just released the option for your client to tip you.
Under the receiving money tab, you can toggle the settings on tipping. By default it is off.
You can use this with any processor and toggle it on or off globally. If turned on globally, tipping can be turned on or off per job, but not for different invoices within the same job.
Then, when the client goes to pay that invoice, if enabled, they are given the option to tip :)