Your Mezu Available Balance is always visible at the top of the home screen once you log into your Mezu account.
The balance is made of funds you've loaded or received. It can also have promo funds that you redeemed on a campaign and funds that expired from transactions you attempted.
When you initiate a transaction and there's not enough balance, that amount will be taken entirely from your default payment method with no effect to your current Mezu balance. For example, you want to pay someone $10 but your balance is only $7. The $10 will be taken from your default bank account or debit card and your available balance will remain at $7.