You chose:

5. Return to the senior salesperson and say that you will follow the norm of the department in order to maintain a good relationship with the other personnel in the department. You believe that now you have to abide by the norm of the team and pad your expense account. Otherwise, you can't continue in your job.

Your acceptance and participation in the practice means that you will be uncomfortable and unable to do anything about it in the future if you gain a management position. This is unlikely to be a practical solution that you can live with.