A friend was just explaining how these work to me, and as I was filling them up I accidentally clicked on extra and it bought me an extra Folder slot.....for 700 SGs. I am super bummed about this, that's a huge amount for something I don't really care for. Is there any way to get things like this reversed or am I just screwed?

Much like any other MMO (Even pay to plays with cash shops like WoW and FFXIV) purchases in-game / etc. are Final.