One of the most common causes for people that hit the ball on the heel of the club rather than on the center of the face of the club is quite simply they stand too close to the ball. Even though they may have very good posture and they may have very good extension of the arms at address, what may happen is that they are standing so close to the ball that their hands are located behind their shoulders. Then as they swing their arms will come out underneath their shoulders and cause the heel of the club to run into contact with the ball. What I would like to see you do as a simple exercise to try to get a comfortable posture is to take your normal set up, with your hands where they normally are, and just position the toe of the club even with the inside edge of the ball. Then just extend the club out until the center of the club is behind the center of the ball. This will get your arms underneath your shoulders and allow you when you get back to impact to produce solid contact rather than those shots that are hit off the heel.