A simple & smart invoicing/billing solution using PHP & MySQL. It enables you to create product family, items, customers, invoices, quotes, & payments.
It also has some great features in which one of the most important feature is client area, you can create users for you customers and they can login to manage their invoices, quotes & payments.
See the demo at:
https://demo.osourcesol.com/smartbilling
user: demo@osourcesol.com
password: admin123