Final Update
 

I just wanted to thank everyone who contributed for all the good advice in this thread. Earlier today, I called Avis and paid the invoice in full. I think I could have eventually won an argument over the diminished value charges (especially after reviewing photos of the damage -- very minor), but I just didn't have the stomach for a big fight.

Lots of lessons learned, though. From here on out, we'll be following jackal's suggestions: For shorter rentals in areas with cheaper rates (such as California), we'll get the LDW. I've registered our business Amex card for the Premium Car Rental Protection plan, which we'll use for other rentals. While that officially only covers actual repair expenses, I've read elsewhere that they can sometimes be of assistance in fighting the extraneous fees that the agencies tack on.