The ornamental design for a selfstandingmilkstoragebag, as shown and described.
FIG. 1 is a front side perspective view of a self standing milk storage bag, showing my new design;
FIG. 2 is a bottom, rear side perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a let side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines in drawings are for illustrative purposes only and form no part of the claimed design. The dot lines depict impression line for tearing of the self standing milk storage bag that do not form part of the claimed design.