IP Purchase issue

I recently attempted to purchase MordeKaiser from last week's free playable characters. I went into the store with 7,500+ IP, clicked to purchase MordeKaiser for 3,150 IP, and left the store with 1,585 IP remaining in my account. I made no other purchases while browsing. Somehow 2,765+ IP was lost in this transaction. I was saving up to hopefully buy a few characters but didn't get past the first one. I would really appreciate an explanation of what happened. Please and Thank you.