Why do companies look for alternatives?

You might adopt traditional billing platforms because they’re easy to start with. But as soon as billing gets complex, it starts showing cracks. Here’s what you run into:

• Rigid contract structures: If you need ramped contracts, mid-cycle renewals, or negotiated enterprise terms, these tools will force you into messy workarounds with multiple plans and manual tracking.

• Unreliable invoicing and reporting: Many teams struggle with inaccurate invoices, inconsistent reporting, and poor visibility into billing data. Explaining bills to customers becomes a recurring pain.

• Weak usage and credit support: Committed usage, pooled credits, or feature-based entitlements aren’t natively supported. Stripe only handles basic usage events and promotional credits, while Chargebee struggles with advanced billing automation, forcing teams to build their own credit engines.

• Opaque systems and poor scalability: Debugging invoices feels like working with a black box. On top of that, implementations are often long, costs are high, and account management doesn’t scale cleanly.

• Compliance and renewal pain: Renewal processes can lead to hidden overusage fees, billing disputes, and a lack of proper contract versioning. Tracking revenue schedules or handling audits becomes a manual burden.

• Developer frustration: Engineers want flexible APIs, but instead end up dealing with rigid structures, incomplete integrations, or clunky dashboards.

The pattern is clear: these platforms work for simple subscriptions, but if you’re building AI-native or agentic products with usage, credits, or feature-based pricing, they quickly slow you down.

Top 3 open-source billing platforms

1. Flexprice

Flexprice is an open-source, developer-first billing and monetization platform. It’s built for modern AI-native and agentic companies like voice agents, video agents, observability tools, and next-gen infra platforms that often hit a wall with Stripe Billing, Chargebee, or Recurly.

Instead of treating billing as a black box, Flexprice gives your team full visibility and control over pricing logic, credit systems, and metering workflows.

Here’s what makes it stand out:

• Transparent and programmable billing – You define how billing works. From hybrid pricing (base fee + overages) to ramped contracts, you model everything as you would any product feature.

  
  

• Native credit wallet support – Flexprice lets you create wallets with recurring credit grants, one-time top-ups, and auto-refill logic. You can set rollover rules, expiry windows, and consumption priority across credits.

  
  

• Powerful metering engine – Meter usage events with metadata (like model=gpt4, region=US, tokens_out=250) and apply pricing rules dynamically. No need to split events by type or manage dozens of separate usage schemas.

  
  

• Feature entitlements baked in – Want to give 50 generative minutes, 1,000 credits, and gated access to premium LLMs in one plan? Define it once with feature management. Flexprice enforces it across your system automatically.

  
  

• Event analytics + usage insights – Get granular breakdowns of usage by customer, SKU, or time window, so you know who’s consuming what, when, and how it impacts your revenue.

  
  

• Built for devs – Lightweight SDKs, async-first APIs, and clean integration patterns make Flexprice easy to embed into your product stack.

  
  

• Self-hosted and SOC2-ready – You can deploy on your infra or use a managed version. Either way, your data stays transparent, auditable, and compliant.

Best for:

• AI platforms

• LLM infra providers

• Usage-based or workflow-based billing

• Companies offering agent tools, credits, or resource limits

Trade-offs:

• Best suited for teams with technical maturity and can be overwhelming if your pricing is seat based. 

2. Lago

Lago is built as a modern alternative to Stripe Billing. It’s best suited for teams that want control over pricing workflows, but don’t want to build everything from scratch.

You can think of Lago as a middle ground: it brings the structure of Stripe-style subscriptions with the openness of self-hosted infrastructure.

Here’s what stands out:

• Event-based metering engine – Lago lets you define custom billable metrics (like api_calls, messages_sent, or storage_gb) and send usage events via API. It tracks consumption against quotas, applies overage rates, and integrates it all into the invoice pipeline.

  
  

• Subscription logic and pricing plans – You can create plans with flat fees, free quotas, and per-unit pricing. It handles upgrades, downgrades, and mid-cycle proration with a subscription model that’s familiar to anyone used to Stripe’s primitives.

  
  

• Hosted UI + Admin dashboard – Lago ships with a web UI for managing customers, usage, plans, and invoices. This is a major win for teams that want non-engineers to manage pricing ops without diving into YAML files or database tables.

Best for:

• SaaS teams migrating off Stripe Billing

• Products with standard usage-based or tiered pricing

• Startups looking for an open-source billing layer without reinventing the wheel

• Founders who want Stripe-style flexibility, minus the vendor lock-in

Trade-offs:

• Limited credit system support – You won’t find native credit wallets or multi-type credit grants like you get with Flexprice. Everything has to be modeled as usage.

  
  

• Workflow assumptions – Lago’s subscription-first design can feel rigid if your product leans more toward one-time grants, prepaid usage, or workflow billing.

  
  

• Advanced features gated behind Lago Cloud – For example, branded invoices, invoicing logic customization, and customer-facing portals aren’t part of the open core.

  
  

• Early ecosystem – Community is growing but still catching up in terms of integrations, SDKs, and plug-and-play tooling.

Lago is a great starting point if your product’s pricing is relatively clean, you want out of the Stripe ecosystem, and you’re not ready to build a fully custom billing stack.

But if you’re building something like GPU usage billing, AI model metering, or flexible credit entitlements, you may outgrow Lago faster than you think.

3. Kill Bill

Kill Bill is the veteran of open-source billing platforms. It’s been around for over a decade and is battle-tested in some of the most complex enterprise billing scenarios— hink telecoms, fintech, and B2B SaaS platforms that need to plug billing into heavily regulated, multi-system environments.

Kill Bill is Java-based, plugin-driven, and comes with a steep learning curve. It rewards teams that need deep customization, not quick setup.

Here’s what makes it different:

• Plugin-based architecture – Every part of Kill Bill is modular. Need to swap in your own tax engine, invoice formatter, payment gateway, or usage calculator? Just write a plugin. The platform is designed to be extended at every layer.

  
  

• Enterprise-grade flexibility – Handles multi-currency pricing, subscription migrations, plan hierarchies, proration, revenue recognition, and parent-child accounts. It supports quote-based sales, multiple payment processors, and even payment retries with retry schedules.

  
  

• Deep catalog modeling – You can build pricing catalogs that evolve over time, with support for add-ons, tiered pricing, entitlements, and contract changes across versions. It’s designed for long-term billing logic that adapts to real-world deals.

  
  

• Auditable + multi-system ready – Logs, data versioning, event buses, and full API access to every object make it reliable in enterprise-grade audit environments. You can integrate it with CRMs, ERPs, and internal accounting tools without fighting vendor restrictions.

  
  

• Community and maturity – Kill Bill has a long paper trail. Docs, migration guides, and community discussions exist—but you’ll need to dig through them. It’s open-source in the true sense: powerful, but you’re expected to figure things out.

Best for:

• Enterprises replacing legacy billing stacks

• Fintech, telecom, or regulated B2B SaaS companies

• Teams with dedicated billing engineers

• Internal billing platforms where every detail matters

Trade-offs:

• High setup and maintenance cost – It’s not a plug-and-play system. Expect to spend time understanding internals, writing Java plugins, and building your own workflows.

  
  

• Not ideal for AI-native SaaS – If you’re billing per GPU second, model variant, or credit wallet, you’ll be reinventing the wheel. It wasn’t built for the pricing logic AI teams use today.

  
  

• Steep learning curve – While documentation exists, the developer experience is dated compared to modern tools like Flexprice or Lago. Expect verbose config, XML, and more boilerplate than you’d like.

  
  

• UI and developer ergonomics – There's no polished UI for managing plans or customers out of the box. You’ll likely need to build one or use a commercial wrapper.

Kill Bill is infrastructure-grade billing: reliable, auditable, and endlessly customizable, but it assumes you have the engineering resources to tame it. If you’re a startup or scaling AI-native product team, there are likely better-suited options. But if you’re running multi-product enterprise billing across regions and systems, Kill Bill might be exactly what you need.

Key takeaways

Traditional billing tools work for simple subscriptions but fall short when pricing gets complex.

If you're billing by usage, credits, or AI model variants, open-source platforms give you the flexibility you need.

• Flexprice is the most modern, developer-first option for AI-native and agentic products.

• Lago offers Stripe-like billing with self-hosting and metering.

• Kill Bill gives you enterprise-grade control, but comes with heavy setup costs.

Choose based on how complex your pricing is and how much control your team wants.

Frequently asked questions

1. What are the best open source billing software solutions available?

For modern SaaS, especially AI-native or agentic products, your top choice is Flexprice. Flexprice leads in usage-based and credit-first workflows. Lago offers a Stripe-like experience with usage metering, and Kill Bill delivers enterprise-level extensibility via plugins.

2. Can open source billing platforms handle usage-based and credit-based pricing?

Yes. Flexprice is purpose-built for such models and includes first-class support for credit wallets, hybrid plans, and eventized pricing.

3. Are these platforms easy to integrate for teams switching from Stripe or Chargebee?

Flexprice will feel familiar if you’re used to Stripe it supports subscription primitives, proration, and usage events with a clean API. Flexprice is developer-friendly with SDKs and async APIs, though its model is more powerful than Stripe.

4. Which billing infrastructure is best if I’m building LLM-based or GPU pricing models?

Flexprice is your best bet. It’s designed to metered price by model, GPU type, or tokens and supports real-time event tracking.