You can make a lot of money if you bill people who are no longer interested in your product and forgot about it a month ago. Sometimes they'll pay you for years without realizing that you provide no useful or necessary service of any kind!
Maybe I'm just not ruthless enough for business, but this does not seem like a great way to make money. I'd rather make less through honest customers who like my product, then screw over those who don't.
Easy fix here: Send receipts every time you bill someone.