In this context it means that they could have used unimportant details to be cruel. When it says "they could have made me pay", pay is not directly referring to money, but to forcing him/her do something they don't want to do.

3. Marked by meanness or lack of generosity, especially in trifling matters. http://www.thefreedictionary.com/petty

When someone is selfish about money, when they save the money too much