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Usually in restaurants where all the of the goods are individually prepare / served.

Many cafes sell also beans and other merchandise that are manufactured or made in batches, and can be $20 per item. So tipping on those items would be similar than you tipping in grocery stores or other stores that sell goods. Like you buy a $20k car, do you tip them 20%?

Personally, I always just change the default calculation and tip on the service items like $1 for coffee/drink.

The parent's point is that you don't normally tip on merchandise you buy. If you buy a beer for $10 and a tshirt for $30 from a bartender, a typical tip would be $2 not $8.

That said, it's sort of an arbitrary custom. It's not very much work to fill a glass of beer, find a t-shirt in some cabinet, or hand someone a cup of coffee.

Not like it makes much of a difference, but in the cases where I've seen it, the merchandise is just on display on the floor. So there is not even an aspect of retrieving it from the cabinet, it is just being rung up.

No, you would tip on the food service amount only. Similary you would not tip at a convenience store or grocery store. At least, this is my US centric viewpoint - other locations might have different customs