How can I automatically send an SMS to customers when their ordered part arrives?

You would need to utilize job tags and another texting platform. I know with Chiirp, you can do something like that. Pretty sure you can do it with go high level also. Jobber does so much, but those job specific automations need to be done by another program. As far as I know, Jobber's automatic texts are mostly for sales and appointments. You may be able to mess around with a certain type of appointment and a tag, but that's the only thing I can think of.