If your balance is below the price of the game it will show that you can’t use your balance. So you are presented with two options top up the balance or pay in full using paypal or credit card. I wanted to pay to use up my balance so topped up now I have to wait up to three days when the guidance on the top up page said it would be typically done in 10 minutes!
Here shows the balance is added within 10 minutes:
Blizzard® Balance is typically available within 10 minutes of purchase.
Here shows its 1-3 days!
Note : To ensure the security of every Blizzard Balance transaction, there may be a delay of up to three days after purchase before Blizzard Balance becomes available for use. Direct debit users can use PayPal as an alternative method of payment.