This question asks about the (mostly?) Ashkenazic practice of giving charity in multiples of 18. Another custom I have observed is to give charity in round numbers + 1 e.g. $51, $101, $201 etc. This practice seems to be prevalent in the Syrian and Moroccan communities.
It looks like a gesture of acknowledgment that the cause deserves more than donation being given I'm sure every dollar helps, but is there a Torah-based source for it or is it purely cultural?