I'm not sure how robust the system is for importing into PostBooks, but if you can mimic the XML exported by Yahoo! that would be awesome. The whole receive an e-mail, drop it into a folder, then import it thing isn't a very good workflow, but it's better than nothing. It'd be awesome if you could do more than one at a time...

Best of luck!

(I'd recommend exporting orders based on a configurable order status. This is how I do it for the EDI module I wrote. Perhaps it creates a unique XML file for each one and just puts them in an FTP folder for you to go download them all from into your local import directory. )