Error in shell script, Mac silent install

I am building an Applescript application to run the shell script for my less-experienced users.  The shell script errors in this fashion:

"installer: Error the package path specified was invalid: '/Volumes/SplashtopStreamer/.Splashtop Streamer.pkg'."

I can sometimes make it go away by mounting the disk image first - perhaps the shell script needs a few second wait between the mount command and the install command?  Maybe you know better.