You gave us no idea of what was on the bill, so it is impossible to say
what you were charged for.

Sure you can dispute it...feel free.

I seriously doubt that it needed spark plugs at 30k miles! Damn near
everything goes 80k to 100k now for a service interval regarding the
spark plugs and other ignition related parts.

It would have set a missfire code if it did need plugs...

I smell a skunk!

Labor is $100+ per hour.  Parts and fluids perhaps $100.

Why in the world wouldn't you get an estimate before they did the work?

I never understood why people with some skill wouldn't do basic crap
such as PVC, air filters, or rotating tires.

For what they did, sounds about right.  Figure they worked on it
probably 4 hrs at $75/hr - $300.  Probably $25-$30 in haz disposal
fees(oil, filters, etc).  Parts will be $100-$150 at marked up
rates(plugs, filters).  Throw in some tax, and it's not inconcievable
to reach $500.

None of this is something you could not have done yourself and saved
yourself $400.  I dont' know why/how they would have screwed up the
tranny doing a flush/change.  They may have started it while it was
empty on fluid and caused it to set a flag.

JW

Unless that's one of the cars that are a right pain in the, er,
tailpipe to service, that sounds high to me.  If you need a factual
basis for formally disputing the claim, check in a flat-rate book.

If they are "very tech. savvy"  why did their work result in throwing a
trouble code and then getting your transmission electronically stuck in
first gear?  One hypothesis on the cost is that they fixed their
screw-ups while the clock was running...

Having vented a bit, let's see what you need to look at when doing your
homework:  parts and labor.  Even seemingly simple vehicles can have
hidden gotchas that make certain items of routine service a pain (those
"five minutes of work -- but an hour to mine down to it and reassemble
everything afterwards" scenarios we all know and love), so maybe tht
would help legitimize the bill, especially around the transmission.
Where are those filters?  Are any adjustments indicated? If this is a
4WD, did they also have to do similar things to the transfer case?

Since you seem to know your way around an engine compartment a bit,
just stare at it and visualize doing each task -- see if something
strikes you, with common sense, as being a lot harder than one might
expect.

Modern cars can also harbor shockingly expensive versions of
functionally familiar parts, so checking the bona fides of the invoice
and (if applicable) preparing a solid dispute would involve pricing
everything they replaced.

This of course assumes they presented you with at least a somewhat
detailed invoice, as most places will do these days.  If they just said
"bend ze knees -- five hundred dollars, please," that itself would make
me go "hmm."

Best of luck,
--Joe