\documentclass{isodoc}
\usepackage{invoice}
\setupdocument{
   ourref = 8234,
     date = 20060401,
  subject = Declaratie verzending aanmaningen,
       to = NTG\\Maasstraat 2\\5836 BB Sambeek
}
\begin{document}
\invoice{
  \itable{
    \iitem{enveloppen}{6,60}
    \iitem{postzegels}{9,00}
    \itotal[Subtotaal]{15,60}
  }
  \\[3ex]\paymentdata
}
\end{document}
