Below is a compact, practical guide to API monetization — commercial tools, open‑source options, integration patterns, and recommendations so you can pick what's right for your product.

1. Core concepts to support monetization

• Authentication & access: API keys, OAuth 2.0, JWTs.
• Metering & billing units: requests, data transferred, compute time, feature flags, seats.
• Plans & quotas: free tier, paid tiers, trial, overage handling.
• Rate limiting & throttling: per‑key / per‑plan enforcement.
• Usage reporting & analytics: per customer, per metric, exportable for billing.
• Billing engine & payments: recurring/subscription, one‑offs, invoices, tax handling, refunds.
• Developer portal & onboarding: sign-up, API docs, self‑service plan changes.
• Automation: webhooks for billing events, metering pipelines for reconciliation.

2. Commercial/hosted platforms (fastest to get started)

• RapidAPI Marketplace — marketplace + developer portal + payment collection.
• AWS API Gateway + Usage Plans + AWS Marketplace — integrates with AWS billing or custom billing; strong for AWS workloads.
• Google Apigee (Google Cloud) — enterprise API management + analytics + monetization features.
• Red Hat 3scale — API management with monetization, integration with payment systems.
• Postman/Other marketplaces — limited monetization but useful for discovery.

Pros: fast setup, billing handled, developer portal and marketplace visibility. Cons: vendor lock-in, cost per request or per user, less flexibility in custom pricing.

3. Open-source / open-core API management options

• Kong (OSS) — high-performance API gateway. Plugins for rate limiting, auth, analytics; monetization commonly implemented by combining Kong (metering/rate limit) + billing provider (Stripe). Kong has an enterprise edition with more built‑in portal/billing features.
• Tyk (Community edition) — API gateway + dashboard; open-core with paid features for developer portal and advanced analytics.
• Apache APISIX — modern, high performance gateway with plugin ecosystem for auth, rate limiting; used with custom billing stacks.
• WSO2 API Manager — open-source, full API lifecycle management with throttling, analytics; often integrated with billing engines.
• Gravitee.io (APIM Community) — API management with policies, developer portal; paid modules add advanced monetization.
• KrakenD — high performance gateway (mostly routing/composition); monetization requires external metering/billing.
• Open-source metering/billing projects — fewer mature, widely adopted full-stack billing engines exist in OSS; most teams pair a gateway with a commercial billing provider or build lightweight billing using Stripe + Stripe Billing.

Pros: lower license cost, full control, self-hosting, highly customizable. Cons: more engineering to integrate payment/billing, developer portal, invoicing, tax, compliance.

4. Billing & payment providers (commonly used with OSS gateways)

• Stripe Billing — subscriptions, metered billing, invoicing, webhook events. Very popular for API monetization.
• Chargebee, Recurly, Paddle — alternative subscription & billing platforms (some include tax, checkout).
• PayPal/Braintree — alternative payment processors in some geographies.

Typical architecture: API gateway (Kong/Tyk/APISIX) → usage metering/logging → aggregator (Kafka/DB) → billing engine (Stripe metered billing or custom) → invoices + webhooks.

5. Developer portal / marketplace options

• Build your own portal (open-source portal templates exist) or use the portal features of Kong/Gravitee/Tyk/WSO2.
• Use RapidAPI or cloud provider marketplaces if you want discovery and external customers without building marketing/portal.

6. Which approach to pick (recommendations)

• If you want fastest time-to-money and low ops: use a hosted marketplace or cloud API management with built‑in monetization (RapidAPI, Apigee, 3scale).
• If you want full control, lower long‑term cost, and can invest engineering time: use an OSS gateway (Kong, APISIX, Tyk, Gravitee) + Stripe Billing + a simple developer portal.
• If you need enterprise features (fine-grained analytics, multi-region, SLA, complex metering): consider Apigee, Kong Enterprise, or Tyk Enterprise.
• If you expect very high throughput and need max performance: choose a lightweight gateway (APISIX, Kong) and handle billing asynchronously via event streams.

7. Implementation checklist (practical)

• Define billing units and pricing model.
• Implement auth & plan enforcement in gateway (keys/tokens + quota checks).
• Implement real‑time or near‑real‑time usage capture (logs → events → metering DB).
• Feed usage into billing provider (Stripe Metered Billing API or your billing engine).
• Build self‑service dev portal (sign-up, keys, docs, plan management).
• Implement invoices, receipts, refunds, tax compliance.
• Add monitoring and reconciliation processes to avoid billing errors.
• Provide trial and easy upgrade/downgrade flows.

8. Example stacks

• Small SaaS: Kong (Community) + Postgres for metering + worker that posts usage to Stripe Metered Billing + simple static developer docs (Hugo).
• Medium scale: Tyk Community for gateway + Gravitee portal + Stripe/Chargebee + Kafka for usage events and reconciliation.
• Enterprise: Kong Enterprise or Apigee for gateway/portal/analytics + integrated monetization + internal billing/invoicing system.

9. Risks & operational notes

• Metering accuracy and reconciliation is critical — design for idempotent events, buffering, and backfill.
• Consider chargeback disputes — keep raw logs for auditing.
• Tax and regulatory requirements vary by customer location — use billing platforms that handle tax (or use a tax engine).
• Security & PCI: avoid handling card data directly; use PCI-compliant processors.

10. Quick list — tools named

• Hosted/commercial: RapidAPI, AWS API Gateway (usage plans), Apigee, Red Hat 3scale, Kong Enterprise, Tyk Enterprise.
• Open-source / open-core: Kong (OSS), Tyk (Community), Apache APISIX, WSO2 API Manager, Gravitee.io, KrakenD.
• Billing/payment: Stripe Billing, Chargebee, Recurly, Paddle, PayPal/Braintree.

If you want, I can:

• Propose a concrete architecture for your expected scale (monthly requests, revenue goals, number of customers).
• Draft an integration plan showing exactly which components to deploy and how they communicate (gateway → metering → billing).