The ornamental design for a posturalsupportcushion, as shown and described.
FIG. 1 is a perspective front view of a postural support cushion;
FIG. 2 is a front elevation view of the postural support cushion; the rear elevation view being a mirror image thereof;
FIG. 3 is a left side elevation view of the postural support cushion;
FIG. 4 is a right side elevation view of the postural support cushion;
FIG. 5 is a top plan view of the postural support cushion; and,
FIG. 6 is a bottom plan view of the postural support cushion.
The drawings are shown broken away to indicate no specific length. The broken lines form no part of the claimed design. The appearance of any portion of the article between the break lines forms no part of the claimed design.