Best open-source alternatives to PayPal

Invoice Ninja 5 is an invoicing application designed for businesses that need to create and manage invoices, support a client portal, and expose an API for integrations. The README presents both a hosted SaaS offering and a self-hosted edition, making it suitable for teams that want either a managed service or full control over their own infrastructure. The project also includes mobile and desktop clients, plus self-hosted deployment options such as Docker and server/VM installation. It documents API usage, developer guidance, and user documentation, and highlights that Pro and Enterprise features from the hosted app are included in the source-available code. The README also shows example login credentials for administrator and client portal access, indicating support for multiple user roles.

BTCPay Server is a self-hosted Bitcoin payment processor designed for merchants and organizations that want to accept Bitcoin without relying on intermediaries or paying processing fees. It emphasizes user control, privacy, and non-custodial operation, with support for SegWit, Tor, Lightning Network integrations, and hardware wallet-based vault workflows. The project is aimed at users who want to deploy their own payment infrastructure or use a third-party host, and it includes documentation for deployment, account setup, and walkthroughs. It also provides merchant-oriented tools such as invoice management, payment requests, point-of-sale flows, crowdfunding, and donation buttons, plus both a newer Greenfield API and a legacy BitPay-compatible API for integration.

Kill Bill is an open-source subscription billing and payments platform aimed at businesses that need to manage billing, payments, and SaaS-style subscription operations. It is positioned as a scalable backend for online businesses, including larger SaaS and e-commerce organizations, and emphasizes avoiding vendor lock-in by keeping business and client data under the operator’s control. The project highlights out-of-the-box capabilities such as real-time analytics, financial reporting, trial management, and promotional discounts. It is described as modular and extensible, allowing operators to disable or replace functionality as needed. The README also points to technical documentation, an AWS one-click deployment path, and hosted sandboxes and developer tools through the project’s cloud site.

InvoicePlane is a self-hosted web application for invoicing and client billing workflows. It is intended for individuals or businesses that want to create, send, and manage invoices and quotes without relying on a third-party SaaS product, while also keeping track of clients, payments, and related financial records. The project is deployed on a web server and configured through a PHP-based setup process. Users download the release, place the files on their hosting environment, set basic configuration values, and then run the installer through a browser. The README also highlights optional URL rewriting via mod_rewrite and points users to the project wiki for installation, upgrades, troubleshooting, and community support.

FOSSBilling is an open-source billing and client management system aimed at online service businesses. It is positioned for web hosting providers, but the README notes that it also fits software licensing, digital downloads, and subscription-based businesses. The project helps automate common operational tasks such as invoicing, payment handling, and client communication. It emphasizes extensibility, multilingual support, and a responsive interface that works on both desktop and mobile devices. The README also points users to a public demo and an installation guide, indicating that the project is intended for self-hosted deployment on standard LAMP/LEMP environments.

Bitcart is a merchant-focused platform intended for users, merchants, and developers. The README presents it as the central repository for a broader ecosystem of related projects, including core daemons, a merchants API, an admin panel, a ready-made store, a Python SDK, and a scripting language for checkout automation. The project is meant to be easy to set up and use, with the main repository serving as the hub for the ecosystem rather than a complete standalone product description. It also points readers to the documentation site and to linked repositories for deployment, administration, storefronts, and developer tooling. The README indicates an open-source project licensed under MIT.

SolidInvoice is an open-source invoicing platform aimed at small businesses and freelancers who need to manage everyday billing tasks. It provides a web-based interface for handling clients, contacts, quotes, invoices, taxes, discounts, and payment collection. The project also exposes a RESTful API for integration with other systems and supports notifications through channels such as email, text messages, and HipChat. It can be installed via Docker, from an archived release package, or directly from source for developers and production deployments.

SHKeeper is a self-hosted cryptocurrency payment processor aimed at merchants who want to accept crypto without intermediaries. It provides an invoice-based payment flow where each invoice maps to a unique address for the selected coin, allowing merchants to track payments by order or external ID. The project is designed for deployment on Kubernetes via Helm and includes guidance for setting up SSL, Lightning LNURL support, and merchant integrations. It also exposes a documented API for operations such as listing supported currencies, creating invoices, checking payment information, handling payouts, retrieving metrics, and receiving callbacks. Ready-made modules are listed for common ecommerce platforms, indicating the software is intended to integrate with online stores.