How to right align a table in a tikz picture

Question

I currently have set up a command that draws a box with a shadow and some text inside that box. I want it to have a minimum-width, but when I set it, the table is now aligned in the center of the box when the table is smaller than the boxes width. I'd like it to be right-aligned. Here's the current setup:

\documentclass{scrlttr2}
\usepackage{tikz}
\usepackage{color}

\usetikzlibrary{shadows}

\definecolor{myblue}{rgb}{0, 0.58, 0.85}

\newcommand*\invhead[1]{\sffamily{\color{myblue}\footnotesize{\textbf{#1}}}}

\tikzstyle{shadedbox} = [
  draw=black,
  shade,
  top color=white,
  bottom color=bottom,
  drop shadow={
    top color=black,
    bottom color=black,
    shadow xshift=2pt,
    shadow yshift=-2pt,
  },
  thin,
  rectangle,
  inner sep=2pt,
  inner ysep=2pt
]

\newcommand*\monobox[2]{%
  \begin{tikzpicture}
    \definecolor{top}{RGB}{250,250,250}
    \definecolor{bottom}{RGB}{235,235,235}
    \node [shadedbox, minimum width=3.1cm] (box) {
      \begin{tabular}{r}
        \invhead{\scriptsize #1}\\
        \footnotesize{#2}\\
      \end{tabular}
    }
  \end{tikzpicture}
}

\setkomavar{fromname}{TestCompany}
\setkomavar{fromaddress}{37 Test Road, Someplace\\Somewhere, 6020\\Australia}
\setkomavar{fromphone}{+61 123 1234 1234}
\setkomavar{fromemail}{accounts@testcompany.com}
\setkomavar{signature}{TestCompany}
\setkomavar{invoice}[Invoice number]{1234}
\setkomavar{date}[Issue Date]{\today}
\setkomavar{customer}[Account number]{1234567}

\begin{document}
\begin{letter}{%
  Mr Nick Sandford\\
  23 Test St\\
  Somewhere, Someplace\\
  Australia
}

\opening{Dear Nick Sandford,}
\monobox{Test Box}{\$0.00}
\closing{Regards}
\end{letter}
\end{document}

I call it like \monobox{This Bill}{\$20.00}. This is what it looks like when it is rendered:

Hopefully I'm not doing anything too wrong :).

Answer 1

To right align text in a node you must set the text width and the the align options on it. In your example code you could modify the monobox command to look like this:

\newcommand*\monobox[2]{%
  \begin{tikzpicture}
    \definecolor{top}{RGB}{250,250,250}
    \definecolor{bottom}{RGB}{235,235,235}
    \node [shadedbox, minimum width=3.1cm, text width=3.1cm, align=right] (box) {
      \begin{tabular}{r}
        \invhead{\scriptsize #1}\\
        \footnotesize{#2}\\
      \end{tabular}
    }
  \end{tikzpicture}
}

The result:

Update: after Martin and Peter's comments I modfied the code, this should work regardless of the width of the input

\newcommand*\monobox[2]{%
 \begin{tikzpicture}
    \definecolor{top}{RGB}{250,250,250}
    \definecolor{bottom}{RGB}{235,235,235}
    \pgfmathsetlengthmacro{\mylen}{max(width("#1"),3.1cm)}
    \node [shadedbox, minimum width=3.1cm, text width=\mylen, align=right] (box) {
      \begin{tabular}{r}
        \invhead{\scriptsize #1}\\
        \footnotesize{#2}\\
      \end{tabular}
    };
  \end{tikzpicture}
}

Remaining problem is that the text is set using invhead and scriptsize and for some reason I can't get TikZ to accept these in the width operation. The manual says "protecting" it with a \noexpand should do the trick, but it doesn't. Therefore too much space is allocated. Anybody have any idea how this can be fixed?

Edit: after Martin's proposal to use a box, why is this not working?

Extra bonus-edit: Martin explained on chat and in this answer (Thank you Martin :)): How can I use an hbox inside a TikZ environment for text dimension measurement?

\newcommand*\monobox[2]{%
  \newsavebox\mybox
  \sbox{\mybox}{%
    \begin{tabular}{r}
      \invhead{\scriptsize #1}\\
      \footnotesize{#2}\\
    \end{tabular}
  }
  \begin{tikzpicture}
    \definecolor{top}{RGB}{250,250,250}
    \definecolor{bottom}{RGB}{235,235,235}
    \pgfmathsetlengthmacro{\mywd}{max(3.1cm,\wd\mybox)}
    \node [shadedbox, minimum width=3.1cm, text width=\mywd, align=right] (box) {
      \usebox{\mybox}
    };
  \end{tikzpicture}
}

This produces a box of the correct size, using the \mywd macro ensures that the text is also right aligned if it's shorter than 3.1cm.