Open source Square alternative

The open source payment processing space has roughly 45-60 viable alternatives ranging from Stripe clones to niche solutions, but most fail because they optimize for developer experience over merchant simplicity. The common failure pattern is shipping robust APIs without solving the actual problem Square solved: making payments frictionless for small businesses that don't want vendor lock-in. Square's real moat was never the code—it was omnichannel integration (POS, online, invoicing) plus physical hardware, wrapped in a consumer-grade interface. Most open source competitors stop at payment processing and ignore the ecosystem. The real opportunity sits at the intersection of three things: (1) merchants actively rejecting SaaS subscription models due to cost, (2) growing demand for on-premise payment infrastructure post-privacy concerns, and (3) the rise of vertical SaaS where indie makers need payment tools that don't take 2.9% + $0.30. A shipper entering this space should focus on either horizontal simplicity (one-click deploy for micro-merchants) or deep vertical integration (e.g., open source + inventory + accounting for restaurants/retail specifically), not another generic payment API.