$600 to fix the spoiler?

Sounds to me like the typical "let's change parts until we fix it" type of troubleshooting that is so common these days. No REAL knowledge, just guesses.

This is why I fix stuff on my cars myself. But then, I fix electronic equipment for a living.

If your battery is over 4 years old, maybe over 3 - you might try replacing it. If it's over 4, replace it anyway -you are on borrowed time.
$89.95 at O'Rielly's