How to include online postage in a scrlttr2 letter?

Another possibiltiy is to use tikz:

\documentclass[a4paper]{scrlttr2}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage{graphicx}
\usepackage{tikz}
\setkomavar{fromname}{John Doe}
\setkomavar{fromaddress}{Fakestreet 1\\123 Faketown}
\begin{document}
\begin{letter}{%
  Jane X\\
  456 Fake Village
}
\begin{tikzpicture}[remember picture, overlay]
  \node [xshift=65mm,yshift=-60mm] at (current page.north west)
    [below right]
    {\includegraphics[trim=57mm 228mm 114mm 55mm,clip]{pstge.pdf}} ;
\end{tikzpicture}
\opening{Sehr geehrte Damen und Herren,}
\closing{Mit freundlichen Grüßen}
\end{letter}
\end{document}

Advantage: Easy to place the postage absolutely.

Disadvantage: You have to compile the document 2 times.

It looks like you can solve this by using KOMA-script options and internal commands. Please, have a look at the following MWE:

\documentclass[addrfield=backgroundimage,%
              fontsize=12pt, UKenglish]%
              {scrlttr2}
\usepackage{babel,graphicx}
\usepackage[utf8]{inputenc}

\setkomavar{addresseeimage}{\includegraphics[%
           trim=5mm 228mm 114mm 55mm,clip]%
           {postage_sample.pdf}}

\begin{document}
\begin{letter}{Name and \\ Address \\ of \\ Recipient}

\opening{Hello}

\closing{Kind regards}

\end{letter}
\end{document}

To change the horizontal and vertical placement of the image, change the first and last parameter (i.e. change the values 5mm or 55mm until it fit).

I have not analysed if this may have unintended side effects.