Front accounting is multilingual and multicurrency software for small businesses.
Front Accounting APIs API is a free Finance API that requires OAuth authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from frontaccounting.com.

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The Front Accounting APIs API is provided by FrontAccounting Wiki (frontaccounting.com).
Yes. The Front Accounting APIs API is listed in the Finance category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Front Accounting APIs API uses OAuth authentication, so you need to register for credentials before making requests.
Yes. The Front Accounting APIs API supports CORS, so front-end JavaScript can call it directly.
Yes. The Front Accounting APIs API is served over HTTPS, so it's safe to use in production.