Hi @AnnL9
I can understand how important it is to know the balance of the billing prepay on your account! I apologize you have not been able to find this as easily as you should. Where exactly were you looking for it at? Though it does not appear on the invoice your balance can be viewed through your My Account section.
In the meantime, being able to see how many months remain in your prepay is a feature request that I have tracked in your account.