How do I combine these two shell commands into a Service?
What about an Automator service like this one:


The payload is the shell script

 for f in "$@"
 do
    pandoc "$f" --css=markdown.css | prince - "$f".pdf
 done


Whatever the selected file or files, the service runs them through pandoc's Markdown parser, then takes the resulting html and processes it with prince, naming the output the original file name with a .pdf extension added.

The piping removes the need for you to manage temporary files.