I claim, the ornamental design for a foldable carton armchair, as shown and described.
FIG. 1 is a top perspective view of the foldable carton armchair, showing my new design;
FIG. 2 is a top perspective view thereof when flattened;
FIG. 3 is a front side elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a side elevation view thereof; and,
FIG. 6 is a back side elevation view thereof.
The broken lines shown in FIGS. 1, 2, and 4 showing of the top border and folding lines of the foldable carton armchair is for illustrative purposes only and form no part of the claimed design.