The ornamental design for shoulder strap for bag, as shown and described.
1. Shoulder strap for bag
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Right
1.5 : Left
1.6 : Top
1.7 : Bottom
The shoulder strap forms a series of elongated and more or less elliptical elements, which on one side of the shoulder strap have an anchor chain link design; the dotted lines in the images represent seams that are not part of the design and are not claimed.