In Ontario Canada there is no provincial tax component on meals costing less than $4. This dates from the time you could get a simple lunch for < $4. Unfortunately it’s never been adjusted for inflation.
No reason not to show amount with tax and give people a pleasant surprise though
Depending upon what you are trying to do, a Progressive Web App (PWA) might be a solution if you don’t mind JavaScript. Avoids a lot of platform learning curve. (But you do end up having to learn HTML and CSS too)