Did you actually notice the room feeling cooler with the heat pump, or was that just a guess on your end? Forty percent less energy sounds nice on paper until you're up at 6am trying to beat the rate hike. That slow cycle would drive me nuts with a family, but if you're home during the day anyway it might not matter. The $19 drop is real money though, so I get why you're torn.

Ngl, that bindery guy gave you a decent tip but there's a catch nobody mentioned yet. PVA alone can work fine for a while, the problem is it never really stops being flexible, so years down the road when it gets cold or dry it can go brittle and crack right along the hinge. The real issue is that straight PVA has no reversibility, once it sets you can't undo it without tearing paper, and it creeps under stress. Most folks who run PVA on linings still mix in a little paste or use a 50/50 blend so it stays a bit softer and can be taken apart later if the book needs a reback. So yeah it feels great on the bench and opens flat now, but I'd keep an eye on that spine in a few summers.