[client] IP price for champion shards

When going to buy a hextech mystery champion shard in the store, I click on it and the box comes up, RP or IP? I have ~1950 IP. Underneath the box, it says you're remaining balance will be 15 IP. I buy it anyways, and now I have ~250 IP. For some reason, when I tried this before my most recent game, I had >1700 IP but could not purchase the shard. It seems to me that the price is correct, but for some reason the store necessitates a higher amount to purchase the shard with IP.