If your studio offers it, you get one email after your practitioner leaves with a link to add a tip. It is optional, and the whole tip goes to your practitioner.
Once your practitioner marks the visit finished, you get an email titled "Add a tip for …". It arrives once. If you would rather not tip, you can simply ignore it — nothing happens either way, and nobody is told.
The link opens a page with a few suggested amounts. Each button shows the actual dollar figure, not just a percentage, so you can see exactly what you are paying before you choose. Some studios also let you type your own amount.
Choosing an amount takes you to a secure Stripe payment page. This is a separate payment from the visit itself, which has already been paid for — a tip is never taken automatically from your card.