The setting of billing alert is usefull for me. But now billing alert just for monthly usage has exceeded. I suggest you to add the billing alert for remaining credit. As example if my remaining credit is $15 then I get email alerts. Because when my credit remaining is $15, then I can predict that my droplet remaining active only for 5 days. This value is obtained from $ 15 / (2 droplet x 24 hours x $0.06). So I should immediately add to my credit.