I also feel this way. For some reasons, I think my natural, quick feeling toward that would be that paying 1.99$ 'seems' more of an annoyance than paying 0$, a

I also feel this way. For some reasons, I think my natural, quick feeling toward that would be that paying 1.99$ 'seems' more of an annoyance than paying 0$, and since it's not enough to be deemed worth the annoyance, I can picture myself putting 0$ instead of 1.99$. For the same products, I feel I wouldn't change the price if it was listed at {3,4,5}.99$, I don't really know why. 1.99$ feels too close to an transaction fee, or some kind of things like that that feels like the money will be lost in The System. While paying >3.99$ seems to have more consistence, more purpose, since my intuition tells me that more of the value will end up in the proper pockets. That's a very subjective take on this, but I support your idea that more people might have paid if the default would have been 3.99$.