Can't use Google Opinion Rewards credits for in game purchase

So i have done a few surveys, earned $5, and i original have $25 in my wallet, and i can clearly see the total balance in google play was $30. But when i make the in game purchase (costs $28), it said i have an insufficient balance of $25.

And yes i used the same gmail account for both. Does anyone know what's going on? How do i use those credits?