If you have money in your HarmonWeb wallet, it will be used for your next bill. This could be for something like renewing a subscription.
On the other hand, if you buy something before any other bills come up, you'll see it as "Credits" in the order summary when you make a payment. If what you're buying costs more than your available balance, it'll be used as a discount, so you'll only have to pay the difference.
Note: If your current balance isn't showing up as credits, get in touch with our Customer Success team. They'll help you out!