\documentclass[12pt]{letter}
\usepackage[businessenvelope]{envlab}
\usepackage[papersize={4.125in,9.5in}]{geometry}

\begin{document}
\PSwait
\startlabels
\mlabel{%
Mr. Magnanimous Understructure\\%
42 Bigfoot Ave\\%
Pittsburgh, PA  15209}{%
E.I.T. Collector\\%
300 Wetzel Road\\%
Glenshaw, PA  15116-2280}
\end{document}