The ornamental design for a packingbox, as shown and described.
FIG. 1 is a front side view of a packing box in accordance with the present invention;
FIG. 2 is a back side view of the packing box depicted in FIG. 1;
FIG. 3 is a left side view of the packing box depicted in FIG. 1;
FIG. 4 is a right side view of the packing box depicted in FIG. 1;
FIG. 5 is a top view of the packing box depicted in FIG. 1;
FIG. 6 is a bottom view of the packing box depicted in FIG. 1;
FIG. 7 is a front side perspective view of the packing box depicted in FIG. 1;
FIG. 8 is a perspective view of the packing box illustrating an example environment; and,
FIG. 9 is another perspective view of the packing box illustrating an example environment.
The broken lines shown in the drawings illustrate environmental subject matter and portions of the packing box that form no part of the claimed design.