> they want to know the posted price, so that they can decide to buy or not
It's trivial to know the price of the coffee with tip: you just add one dollar. To make it sound like this is task is so difficult OP would sooner not buy a coffee seems disingenuous to me.
For your convenience, I've included the top two definitions of "disingenuous" below. I think the second one is particularly apt.
1. Not straightforward or candid; insincere or calculating.
2. Pretending to be unaware or unsophisticated; faux-naïf.
> Just price in the tip exactly like you would for going out to a restaurant.
You tip 20% at Starbucks window?