In some languages such as Hebrew, there are no special characters for numbers; the same symbols that are letters of the alphabet are used for numbers. In Hebrew, the number “18” uses the same characters as the Hebrew word “l’chaim”, or “life”. It’s common among Jews to give gifts that are 18 dollars or multiples of 18.
So if a Jew gives you a gift of $36, he is wishing you “two lives”, or “long life”. As a wedding gift, this might mean “a life for both of you”.