Given that the idea of adding a new notification for remaining credit was declined a few times, can you add the remaining money to monthly usage notifications?
In my use case, I put 25 USD in my account but the droplet uses only 5 per month. So I'm covered for 5 months, but usually I don't remember to add more credit. If I use monthly usage notification with a value of 25 it'll never notify me because I never use more than 5 USD per month, on the other hand if I configure the monthly alert to 5 USD i get notified all the months... but I still have to remember when my 25 USD ends (that's what I need: to know when I need to put more money).
So, if the remaining money is added to the existing one, then it may serve for both purposes.