The difference in the construction of the signs ka and ša are as follows: "ka" when scribed in the Amarna letters often shows the distinctiveness of the right section of the sign, versus the left section.
For ša, the right section is constructed with two wedge strokes (one scribed above the other), between the two verticals, at right.
(Note, the ša of EA 365 appears to have 3-horizontals at left (differing lengths), then the 2-verticals with the 2-wedge strokes, at right.)
The usage numbers for ša in the Epic of Gilgamesh are as follows:[3] ša-(66) times.
Since it is similar in appearance to cuneiform ka, the large difference is that ka can easily be found as a suffix to words, for example in the Canaanite sub-corpus of letters as "Servant-Yours", , (ARAD-ka).