If you map the letters of Hebrew to their numeric equivalents (analogous to A=1, B=2, C=3, etc.), the word C’hai, which means “life”, has a value of 18. For this reason, giving thing in multiples of 18 is considered a way of wishing the recipient life (or long life, or good life)
-Muttrox, who is such a lapsed jew he is not at temple on this most holy day but instead at work and surfing SMDB