The USPS settings can accessed from the Settings panel, which is at the far right of the main menu.
Quick Note: Your USPS settings may need to be enabled first.
I will now go top-to-bottom an cover all the fields in the USPS Settings.
The first area that you will is the top-up window. You will see your credits total here and will be able to top-up using your credit card.
Before topping up a pop-up window will appear reminding you that this will charged from your default card that you added in your subscriptions settings.
Bellow that you will find all of your credits history. How much you topped up, and when you spent it.