Just as there should be a balance between chest and back training to avoid asymmetry and compensations, there should also be a balance between training the upper and lower traps.
Most people with poor posture involve their upper traps way too much in movements where they really shouldn't be overly involved. You can counter this and beef up the mid back in the process by tapping into the lower traps. The single-arm trap-3 raise does a nice job in targeting the lower traps.
Want to take things up another notch? Try the kettlebell angled press.
Kettlebell Angled Press
This movement still takes advantage of the proper force angle, but it encourages an isometric contraction of the shoulder retractors and postural muscles because the bell is being held in close to the body. In other words, you're not allowing gravity to pull the bell down towards the floor. In addition, the lever arm created by the arms sliding forward and outward creates plenty of work for the thoracic spine extensors while making sure the delts are less involved.
The fact that you're using a kettlebell instead of a dumbbell is a big deal. The weight distribution of the bell makes the weight feel that much more daunting at the top of the force curve. It's a subtle change but it makes a big difference.
The best part about the kettlebell angled press is that it encourages a rigid trunk and spine. Because you're carrying the kettlebell in both hands, the torso isn't allowed to "rest" at all. That activates many more muscles to encourage good, even spine posture through the entire back.