We have tried the plugin and though it works fairly well, unfortunately there is no way to have people add a tip without it being taxed - unless you set it up as a service product and set no taxes on service products. Unfortunately, we sell to-go food - so all our products have to be service products - not physical - since Squarespace requires physical products to have shipping info.
We received complaints from customers that their tips were having sales tax added, and furthermore, this is technically not even legal in California.
Maybe Squarespace will eventually figure out a way to implement this feature? We have been working on it for over a year.