The X and Y shift option allows you adjust the placement of your mark on the map. Since the marks are placed by True Type Fonts, the map browser will attempt to center the mark on the Zip code. However, if you select a mark such as a flag and you want the base of the flag (not the center of the flag) to be centered on Zip code area, then you will need to adjust the mark up accordingly.
The 0,0 position is in the top left corner. Positive X values move the mark right. Positive Y values move the mark down. Also, some font marks exist above and below the center of the space consumed by the font. The period . for example, may need to be shifted up (depending on the size of the mark), since the center of the font space is not the center of the dot.
The Florida Keys and the Chicago area are good places to check the alignment of your font.