Might be a bug with the store.

So I just recently bought Wukong today, however the bar where my RP and IP is showed I have -349 RP. Wukong was 880 RP, I spent $10 to get 1380 RP and I also had 31 RP already so that 1411 RP. I may not be a mathematician, but I should definitely have 531 RP....not a negative amount. Probably maybe might wanna look into that, Riot.

EDIT: It also even said in the little purchase window before you actually make the transaction that I would've had 531 RP if I used said 1411 RP. It did just fix itself after I closed out and logged back in. Still might wanna look into that.