The ornamental design for a postureseat, as shown and described.
FIG. 1 is a top perspective view of a posture seat showing the new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
Surface shading is used to shade FIGS. 1-8 to show clearly the character and contour of all surfaces of the 3-dimensional aspects of the design and form a part of the design sought to be protected.