\documentclass{scrlttr2}
\LoadLetterOption{${LETTER_OPTION}}

\setkomavar{subject}{${SUBJECT}}

\setkomavar{yourref}{${YOURREF}}
\setkomavar{yourmail}{${YOURMAIL}}
\setkomavar{myref}{${MYREF}}
\setkomavar{customer}{${CUSTOMER}}
\setkomavar{invoice}{${INVOICE}}
% \setkomavar{yourref}[alternative name]{value}

% custom date
%\setkomavar{date}{}

%\setkomavar{signature}{\electronicsignature}

\setkomavar{specialmail}{${SPECIALMAIL}}

\begin{document}
\begin{letter}{${ADDRESS}}

    \opening{${OPENING}}

    ${BODY}

    \closing{${CLOSING}}

    \ps{${PS}}

    \encl{${ENCL}}

\end{letter}
\end{document}