Update, almost 2 months later:

Complete radio silence from Avis. I have called their "customer relations" number twice since I lost the key and had to have the rental car towed, and they advised me I'll get a bill for the towing and replacement key in my email once everything has been reconciled on their end.

I checked my credit card statement and the original $149 for the rental itself is all that Avis has charged me. (At least they logged the "return" at the right time and didn't charge me for any extra days!)

So...I'm just waiting on the other shoe to drop, so to speak.

Incidentally - the friend I was staying with in Connecticut ended up finding the key fob, about a week after I left. It had fallen into his garden box, right in front of his front porch. It must have fallen out of my pants pocket and somehow gotten buried/obscured by dirt or plants. He's held onto it but I doubt Avis would ask for it back at this point - they've surely re-keyed the vehicle by now.