The ornamental design for a flatitemseparator, as shown and described.
FIG. 1 is a perspective view of the flat item separator showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a one side elevational view thereof (the other side view is a mirror image of FIG. 6);
FIG. 7 is another perspective view of the flat item separator; and,
FIG. 8 is a further perspective view of a flat item separator.
The broken lines showing a panel on the separator arm, paper, a hand, and the bottom surface in FIGS. 1, 2, 3, 7 and 8 are included for the purpose of illustrating environment and form no part of the claimed design.