Which Billing System for VPS and Dedicated Server

I am looking for a billing system with nothing similar like WHMCS or HostBill. Something like what OVH public cloud, Digitalocean and Hetzner Cloud provide to their frontend. We can integrate our solutions on our own but cannot find any billing system that have this type of billing system. The one where I can manage authentication elsewhere and customers can order services after they Login with some clicks only. Also, Postpaid is a priority because even I know, that everyone likes postpaid!

Any recommendations??? (Kinda billing system that can be further extended as we like)