Is that correct to say "Gracias, guapa" or "Gracias, cariño" to Spanish people that you don't know? E.g. when you are grateful to a waiter, can you say something like this? I've heard it many times but I'm not sure about the context.
There are two issues here: Generation and Gender
An older woman might say guapa or bonita to a younger woman she does not know when a service or favor is rendered. She might also say guapo to a younger man. I am over 60 and can see myself saying it if the young man has been especially nice to me.
A man should be careful to whom he says this. Let's say an older guy is in a restaurant where he knows the waitress, he might then venture a guapa. But it must be used with care.
If a young man says it to his contemporary, it can be flirting. Yes. If the young man is pretty sure of himself and says it with a smile to a woman his own age, it can be okay.
What is to be avoided is old(er) men just throwing it out there all the time.
Also, cariño is more intimate and usually used with people you know well, friends or family members. It would not be used with "the general public" by either sex.
spanish native here.
Avoid doing that in the majority of the cases, because if you don't know the other people, they would think that you are flirting with them. Of course this goes with the right context.
Personally I would not, unless I'm totally sure that the other person would not care, or care positively. I mean to be confident with the people I'm talking about.
Just a simple "Gracias" (Thanks) or "Muchas gracias" (Thank you very much) is quiet enough. You could extend the vocals as in "Muchas graciaaas" to express it in a more meaninful or emotional way, without sounding like harrassment. Also, you could said "Muchísimas gracias" which translates to "Thanks a lot".
NUNCA y de ninguna manera debe decir tales expresiones a una persona que recien conoce y mucho menos si es a un mesero/a o una persona que lo está atendiendo.
Esas expresiones están reservadas para dirigirse a una mujer con la que se tiene alguna relación sentimental.
Sencillamente se dice "gracias señor" o "gracias señorita", o "muy amable por el servicio"-