Seeing as there's the current sale going on, I wanted to buy the game Mountain, mainly because it was $0.18. However, whenever I go in to pay for the thing, it gives mean error message saying "Due to processing fees the minimum amount is 100 cents" and won't let me check out. Is there any way for me to buy it at the listed price or should I assume that it's more of a pack-in title with the bundle that it's a part of? I'd assume that this probably affects a lot of games that cost less than $1 too.

Yes  you need to buy another game or more that will be  $1 or donate to the developer. i dont know why  some developers lower it beyond less than 20  cents since that is just basically makes the game free in every nation but maybe i think they mistaken the 80% as $1.

It is odd that the only way you can buy a bunch of games from different developers in one go is if they put them in a bundle, and not if you put them in a basket.