What is the ruling on helping a company that produces lettuce carry bags with the name "Christmas" on them, and is that considered disbelief (kufr) or a major sin, and is it permissible to assist in that matter?

If the question is about placing promotional stickers for Christmas on bags of lettuce or salad, then it is not permissible to do so because it is assisting in evil. If it is possible to take leave during the holiday period to avoid this, then it is a good solution. However, if that is not possible and you are in need of work and cannot find any other, then there is no harm in remaining in it until Allah facilitates another job.