Stripe.net is a sync/async .NET 4.6.1+ client
Laravel Cashier provides an expressive, fluent interface to Stripe
Accounting & Best Practice Business Administration System
SmartPOS (Includes->SmartERP distro of Idempiere ERP) is 100% Web
Invoice and office utilities program for customer managment
Carting Management System
JavaPOS implementation and more, for Brazilian Retail applications
Captures time & expense by project and account; generates invoice
A complete invoice managment solution for Elastix PBX
Free web-based invoicing primarily for Finnish companies
TOSPMS - A practice management system for the healthcare industry.