This posture gives the appearance that you're standing your ground, not willing to give. Soften your posture - lower your arms to just above the waist with one arm out of the fold and laying on top of the other arm, not tucked in. When your left arm is across your waist with the right elbow resting in your left hand with your chin resting in your right hand, you appear to be concentrating or thinking. Eye contact and smiling is nonverbal. It can draw a customer to your booth.
People like to be noticed.
Trying to smile constantly will appear to be artificial at times. It isn't necessary to have a big smile on your face all day, but give the illusion of one. You can feel your face loosen and your mouth come up at the corners. Fold your arms in across your chest now, make your eyes smile. You can feel your arms loosen and body start to relax.