Why is the Hebrew letter "shin" on the front of the mezuza cover?
"Shin" is the first letter of one of G d's names, "Shad-dai" - Almghty. It is also an acronym for "Shomer Delatot Yisrael - Guardian of the Doors of Israel." Thus, it is appropriate to be on the mezuza cover. In addition to being on the mezuza cover, the word Shad-dai is also written on the back of the mezuza parchment itself.