The problem with comparing knuffel to Gaia's Halo is that the halo is an item and, as others have already said, knuffel aren't meant to be at the same level as items.
I think that perhaps it might be a good idea to have a price cap for the knuffel. Maybe they can't be higher than 10 fc or something like that. If people, especially newcomers, see that you're openly setting the price for something at 16+

they're going to be very upset about it. 10 fc would still be enough so that most people wouldn't buy it, but it wouldn't look as greedy.
I have an idea that might boost donations, but I'll post it later as it's already late.