FIG. 1 is a top front perspective view of the post cap of the present design; FIG. 2 is a bottom front perspective view of the post cap of FIG. 1; FIG. 3 is a front elevation view of the post cap of FIG. 1 (side and rear views are identical); FIG. 4 is a top view of the post cap of FIG. 1; and, FIG. 5 is a bottom view of the post cap of FIG. 1.