My personal experience, with a job that is only sort of customer facing, is that an email/note to the supervisor is the best. I do really enjoy and appreciate personal notes of gratitude and the occasional gift (it does make my day) but let my boss know, they’re the ones who decide on promotions/raises, etc. People are quick to complain to the “manager” about bad service, we should be just as quick to let the manager know about good service.