I have installed API Manager & DAS server. I want to install billing system. Is there any link which will help me to integrate or install billing system with wso2?
Yes you can use third party billing systems like zuora or coral and plug it to API Manager to do your billing. You can simply do this by using API_REQUEST_SUMMARY table generated in DAS, and send that API level data to your billing engine to bill per API.
If you want to configure API Monetization please refer [1]
[1] - https://docs.wso2.com/display/AM1100/Configuring+API+Monetization+Category+Labels