Most European e-invoicing tools fall into two camps: heavyweight compliance suites priced for SAP customers, or fragmented open-source libraries with stale dependencies and no support. InvoiceXML is the developer-first middle ground: - One API, every major European format. Factur-X, ZUGFeRD, XRechnung, UBL, and CII. - Generate, validate, and convert between them through a single REST endpoint. - Human-readable errors built in. - AI-assisted PDF prefill. - Zapier, Make, PowerAutomate ready