This is just a suggestion to introduce you a new feature.
So if you open the Billing page (https://cloud.digitalocean.com/settings/billing) you will have a section called "Your Credit", my suggestion is create a similar section like "Billing Alerts" but with the "Your Credits" value, when this values is lower than "X.YZ money" the user receives an email with this alert.
For me this makes sense because usually I do not visit the site a lot.
We do send an automatic email alert to users when their credit expires. We hope this helps in staying on top of credit consumption.
Thank you for your feedback unfortunately we can not implement every feature request. We do have the billing alerts that you mentioned. You could also set that to $1 and then you would receive an alert when basically all of our credit runs out. Not ideal, but a potential work around. Thank you