FIG. 1 is a front perspective view of the shoulder bag. FIG. 2 is a front view of the shoulder bag. FIG. 3 is a rear view of the shoulder bag. FIG. 4 is a left view of the shoulder bag. FIG. 5 is a right view of the shoulder bag. FIG. 6 is a top view of the shoulder bag; and, FIG. 7 is a bottom view of the shoulder bag. The broken lines are directed to environment and form no part of the claimed design.