Amazon coins a scam or am I just dumb?

You have 1,030 Amazon Coins.

Okay I have 1,030 coins. I want to unlock the last two wings of Karazhan. 2 wings is $9.99. $9.99 is 999 coins. Yet when I go to buy the last two wings it wants to make me spend more money saying I need 4 more coins? Uhh no i don't. I have more than 999 coins. WTF is going on? Feel like they stole my money.