Kill Bill handles the complete subscription billing lifecycle from plan definition through invoice generation to payment collection across any gateway, with 15 years of Fortune 500 production use and 5,600+ GitHub stars behind it. Plans, trials, add-ons, and pricing tiers live in a versioned XML catalog that propagates changes without code deploys. The multi-phase subscription engine manages trial periods, promotional discounts, evergreen billing, usage-based metering, and hybrid pricing models with automatic proration on plan upgrades and downgrades. Automated invoice generation calculates charges, applies credits and adjustments, and triggers payment collection through pluggable gateways including Stripe, Adyen, Braintree, PayPal, and CyberSource. The Java plugin framework extends every billing domain with custom logic for tax calculation via Avalara AvaTax or Vertex, fraud detection through Accertify or Feedzai, and ERP integration without modifying the core. The Kaui admin dashboard gives finance and operations teams account management, subscription controls, invoice views, payment configuration, billing timeline visualization, MRR tracking, advanced search, and full audit history with object-level change tracking. Multi-tenancy isolates billing data across thousands of brands with tenant-specific catalogs, payment configs, and RBAC policies. REST APIs expose every operation with webhook notifications for downstream integration. Deploys via Docker Compose on MySQL or PostgreSQL. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. Apache 2.0 licensed.

Hyperswitch from Juspay delivers the most comprehensive open-source payment orchestration stack available — a Rust-built platform that connects to Stripe, Adyen, Braintree, Worldpay, Checkout.com, Cybersource, and 120+ payment processors through a single API integration. Intelligent routing drives each transaction to the processor with the highest predicted authorization rate using rule-based, volume-based, and machine learning algorithms, while seamless failover and automatic retries recover revenue from declined transactions using 30+ parameters including decline codes, card BIN, transaction size, and region. The PCI-compliant card vault stores payment methods securely in a dedicated service separate from the main application server, and the reconciliation engine automates 2-way and 3-way matching with backdated support, staggered scheduling, and customizable audit-ready outputs. The Control Center dashboard provides visual workflow builders for routing rules, payment analytics, and operations management. Cost observability tools drill into scheme, interchange, and acquirer fees to detect anomalies and optimize processing costs. The modular architecture allows deploying individual components — routing, retries, vaulting, observability, or the full suite — without vendor lock-in. The web client SDK collects payment information with prebuilt UI components and supports WooCommerce integration. Deploy via Docker Compose with the app server, control center, card vault, and PostgreSQL. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. Apache-2.0 licensed.