We build all our tools from customer support, CRM, billing, payroll to accounting -> https://erpnext.com. Its also open source, and would love more startups to use it. https://github.com/frappe/erpnext

The benefit of open sources is that you can build deep integration to your processes and also grow it as your company grows. With closed tools, you will end up paying a lot and also lose the opportunity to drive more team productivity by being unable hack and extend your tools.