StackBill - Cloud Management Portal (CMP) on Apache CloudStack ®

HostBill | Billing & Automation Software for WebHosts (hostbillapp.com)

WHMCS | Web Hosting Billing & Automation Platform

Ubersmith: Subscription Business Management

Managed Cloudstack - Billing, Security & Optimization | Apiculus

Billing and Support Solution for Apache CloudStack (stackconsole.io)

 

I’m not sure you’re looking for a fully-featured portal, so maybe something like this would work: Usage-Based Billing - LogiSense?