As far as I know you get to submit your apps on their Amazon Developer Console for one year free, then it's $99 per year. Does anyone know if they still charge this annual fee? I can't find any recent info on this and my one year is coming up.
For quite a while they kept saying the fee was being waived 'for now', but I recall reading somewhere not too long ago that they have completely gotten rid of the fee. I've been selling on Amazon for a few years now and they never did charge me any fees anyway. I think they were just keeping their options open and finally nixed the idea altogether.