https://maps.app.goo.gl/mFpkBuEgPhrCAuNN8
Looking at photos of their menu, they don't have a minimum gratuity specified. If this is a recurring issue, they should specify the expected gratuity on the menu.
Or even add it automatically for people, for your convenience a 18% gratuity will be added to all checks.
They are the business owner, they have full control of how these interactions go.