Open-source
InvoicePlane: An open-source, free invoicing system for teams
InvoicePlane is a self-hosted, free, and open-source clients, invoices, and payments management system. It is a simple useful app for freelancers, small and medium-size businesses. The projects got more than 100,000 downloads almost from all countries. If you are a team of two or more two members, InvoicePlane comes