Joseph,
What’s the big deal?
Hashem’s honor.
Tipping is very nice, it makes a kiddush hashem, its very much appreciated, but who says one is required?
Ask your Rov if, when faced with the choice between making a kiddush Hashem and a chillul Hashem, are you required to make the kiddush Hashem?
On many other topics, you come down on the strict side. Why, however, when it comes to your gelt, does derech eretz become optional?