This is the cheapest I have ever seen these. I know they dropped to 12/12.50 ish on ebay but if you are prime you can get them £9.99 all in and delivered. I can't see non prime as I logged out and it still looks the same.
I use them in stray cat boxes they are fab and last a good 8 hours even in minus temps. Longer if the pet is cuddling it. They are hard discs but have a soft fleece cover. I know people who use them for dogs, cats, rabbits, guinea pigs and even as an alternative to hot water bottles if you have bad hands/are elderly etc.
I know its not a massive saving but when you have several pets or donate to the shelter like i do it can add up. First time posting on the app so hopefully I've done it right, apologies if not.