\documentclass[11pt,twoside,a4paper]{article}
\usepackage{a4wide,amsmath,amssymb}
\usepackage{invoice}

\begin{document}

\begin{invoice}{EUR}

\ProjectTitle{Monat X}%
% Foo Bar Baz

\Fee{Datum} {Stunde} {10}

\end{invoice}

\end{document}


===

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VAT is not zero!

! Missing number, treated as zero.
<to be read again>
                   S
l.12 \Fee{Datum} {Stunde} {10}

? X