invoicing

GetPaid (Formerly the Invoicing plugin) is a lightweight Payments and Invoicing system for WordPress. It can be used to sell anything online via payment forms or buy now buttons that can be added to any landing page. It can also be used by freelancers to manage their Invoices or by 3rd party Themes and Plugins as their payment system. GeoDirectory currently uses GetPaid as its payment system. Maintained by AyeCode on GitHub, where it has earned 43 stars from the community.

It's actively developed around accounting, invoicing, payment, and is a solid reference for anyone building with these tools.