How to: Payment and Billing Integration
* [Adapty.io](https://adapty.io/) – One-stop solution with open-source SDK for mobile in-app subscription integration to iOS, Android, React Native, Flutter, Unity, or web app. Free up to $10k monthly revenue.
  * [CoinMarketCap](https://coinmarketcap.com/api/) — Provides cryptocurrency market data including the latest crypto and fiat currency exchange rates. The free tier offers 10K call credits/month.
  * [Currencyapi](https://currencyapi.com) — Free currency conversion and exchange rate data API. Free 300 requests per month, 10 requests per minute for private use.
  * [CurrencyApi](https://currencyapi.net/) — Live Currency Rates for Physical and Cryptocurrencies, delivered in JSON and XML. The free tier offers 1,250 API requests/month.
  * [CurrencyFreaks](https://currencyfreaks.com/) — Provides current and historical currency exchange rates. Free DEVELOPER plan available with 1000 requests/month.
  * [currencylayer](https://currencylayer.com/) — Reliable Exchange Rates and Currency Conversion for your Business, 100 API requests/month free.
  * [exchangerate-api.com](https://www.exchangerate-api.com) - An easy-to-use currency conversion JSON API. The free tier updates once per day with a limit of 1,500 requests/month.
  * [FraudLabsPRO](https://www.fraudlabspro.com) — Help merchants to prevent payment fraud and chargebacks. Free Micro Plan available with 500 queries/month.
  * [FxRatesAPI](https://fxratesapi.com) — Provides real-time and historical exchange rates. The free tier requires attribution.
  * [Moesif API Monetization](https://www.moesif.com/) - Generate revenue from APIs via usage-based billing. Connect to Stripe, Chargebee, etc. The free tier offers 30,000 events/month.
  * [ParityVend](https://www.ambeteco.com/ParityVend/) – Automatically adjust pricing based on visitor location to expand your business globally and reach new markets (purchasing power parity). The free plan includes 7,500 API requests/month.
  * [Qonversion](http://qonversion.io/) - All-in-one cross-platform subscription management platform offering analytics, A/B testing, Apple Search Ads, remote configs, and growth tools for optimizing in-app purchases and monetization. Compatible with iOS, Android, React Native, Flutter, Unity, Cordova, Stripe, and web. Free up to $10k in monthly tracked revenue.
  * [RevenueCat](https://www.revenuecat.com/) — Hosted backend for in-app purchases and subscriptions (iOS and Android). Free up to $2.5k/mo in tracked revenue.
  * [vatlayer](https://vatlayer.com/) — Instant VAT number validation and EU VAT rates API, free 100 API requests/month

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