Where to put “per fax/mail in advance” in scrlttr2?

Question

If you send something with a formal/legaö letter, you may provide a copy via mail or fax or so before. If so, you usually change that copy to include something like this:

per fax followed by letter
per mail followed by letter

or:

per fax in advance
per mail in advance

If I style my letter with scrlttr2, where do I put that?

The receiver address field is bad to abuse and I only found the specialmail KOMA variable. However, this includes it at the top right of the address field and underlines it. I guess this is supposed to be used for something else. So could you please also explain to me, what I should use specialmail for?

What I expect

As far as I have seen in most letters these words are printed after or – more often (and I would prefer that) – one or two lines before the receiver's address. The text is usually written -- surrounded by two or three hypens --.

So in the end, it may look like this:

-- per mail in advance --

Don Joe
c/o Some Company Org
Goodstreet 3
0283 City
Germany

Subject: Demonstration for StackExchange

Dear Mr. Joe,
[…]

Answer 1

If you do not use KOMA variable location yet, you can set this variable to

\setkomavar{location}{-{}- per mail in advance -{}-}

Then you can adjust the position of the location field to your needs, eg:

\setplength[-]{lochpos}{\useplength{toaddrhpos}}
\setplength{locvpos}{\useplength{toaddrvpos}}
\setplength{locwidth}{\useplength{toaddrwidth}}
\setplength[2]{locheight}{\baselineskip}
\addtoplength[-2]{locvpos}{\baselineskip}

Example:

\documentclass{scrlttr2}

%% to show the position of fields address and location:
%\LoadLetterOption{visualize}
%\showfields{address,location}

\usepackage{blindtext}% only for dummy text

\setkomavar{fromname}{rugk}
\setkomavar{location}{-{}- per mail in advance -{}-}

% change the position of the location field
\setplength[-]{lochpos}{\useplength{toaddrhpos}}
\setplength{locvpos}{\useplength{toaddrvpos}}
\setplength{locwidth}{\useplength{toaddrwidth}}
\setplength[2]{locheight}{\baselineskip}
\addtoplength[-2]{locvpos}{\baselineskip}

\begin{document}
\begin{letter}{Don Joe\\c/o Some Company Org\\Goodstreet 3\\0283 City\\Germany}
\opening{Dear Mr. Joe,}
\blindtext
\closing{Best regards}
\end{letter}
\end{document}