I have $30.81 on my account and i cant buy a $29.99 add on?

I am trying to purchase Call Of Duty Black Ops 3 Chronicles for my son. He has $30.81 on his account and the game is $29.99. Whenever he tries to buy it he gets the screen that says, "How do you want to pay the rest?" I am confused and he really wants this
game but I don't want to buy another $15 xbox card for him and have it go to waste. If you could help me that would be greatly appreciate. Thank you.

:)

 

Destiny rise of iron I can't buy it

so I got 31 dollars on my Xbox account and rise of iron on Xbox cost 29.99 so the tax is 30 I press buy it tells me how do I wanna add more what do you mean add more don't I have enough if its 29.99 the next its 30 dollars and I got 31.84 in my Microsoft
account how much more do I need to buy It pls help or help by telling me Thebes's full price of it or tell me what I'm doing wrong because I don't wanna a look like I wasted money for no reason thanks in advance

 

Do I have enough for this game?

So I'm thinking of buying two $15 Xbox cards to buy this $29.99 game on demand. However, since it requires tax, I only have $0.28 in my account. Would I have enough to buy the game with tax?

 

Probably you forgot the tax?

That is added on top of the $29,99

 

Yeah, I get that too - can't use 80cents leftover in my account... I think they are offsetting the local currency conversion within our Microsoft accounts, so up o 1 to 2 dollars is not usable, based on your countries conversion rate against the US dollar.