I hate it when girls say that. I'm a counter. I notice things that occur in patterns. 78% of check out girls at stores end the transaction with " have a good one. "
It bugs the shit out of me. It implies apathy, as in the girl is generally disinterested in whatever that " one " may be.
It's sounds unprofessional, as if they are unable to distinguish between afternoon or morning. It would sound better if they said, " have a nice day "
Could just be one more sign of the apocalypse.