Probably this https://invoice-generator.com/developers. More on it here https://github.com/Invoice-Generator/invoice-generator-api.

This appears to be a free service, but the code behind the API is not available.

Although, the one benefit for InvoicingAPI is that it gives an option to quickly give a payment code instead of having to send them to another payment application.

FreshBooks has a similar API but different pricing and far more features. Payments integrated too.