At the top of the right-hand frame, set the first drop-down to files or folders, then save your Service as Print File (or whatever you want to call it). You can now print by selecting a file and choosing Service » Print File, and you can optionally assign a keyboard shortcut using the Keyboard Shortcuts tab of the Keyboard System Preferences panel.
You can also download the finished Service, if you prefer.
[robg adds: I'd suggest setting the second drop-down menu to Finder, as that's where it makes sense to be selecting files for printing. I tested this (by building it myself, not by using the downloaded file), and it works -- note, though, that the file will print immediately, without any confirmation or settings panel, nor with any ability to select which printer will be used.]

