Services such as shutterfly use chemical processing with printers such as AGFAs or Frontiers. You will need to use the Contone (continuous tone) setting for those printers.
Don't worry about the native printer resolution. Just crop and size the image without resampling. Whatever dpi you get after that is what you should sharpen for. For example, after changing the image dimensions (again, without resampling so the image pixels aren't changed) if the dpi reads 240.555 in the image size dialog, then you'd set PK to Contone, 250dpi.
