DesignGuy writes:

Cost (parts, labor, installation) was $825. This seems high but wanted to
check to see if in fact it is.

You can find marked-up prices for parts at grainger.com.

Your tech is grossing about $500 profit for himself or his boss.

Also, was told two things can cause corrosion of coil:

Research "formicary corrosion".

Failures in the 5-year and up ages are surprisingly common. It's not your
fault.