Whenever I have a store employee end my transaction with a generic "Happy Holiday" at this time of the year, I have a perfect opportunity to remind them (one on one) that there are several holidays observed at this time of the year, but the one I am celebrating is Christmas. Depending on their response, it may be appropriate to remind the individual that Jesus loves them and express empathy that their employer is not allowing them to express their personal joy at that fact. I don't think it is preventing communication of the message of the Gospel at all. We need to be creative in our counter-tactics, not angry!
Excellent.
Yes, Joyous, not angry!
I just respond with "Merry Christmas" and this usually gives said checkout clerk a bad case of saucer eyes. The clerk has no power over what she can say unless she wants to be fired.