Throughout this guide, you'll see how to disable the automatic use of your account balance to clear all the unpaid invoices.
Out of convenience, the default behaviour is to be enabled so all the invoices are cleared automatically upon adding funds.
To deactivate the automatic credit clearing process, you would need to go to your client-area under the "My Invoices" section.
At the bottom of the sidebar titled "Billing", you should see a menu titled "Disable Automatic Credit Usage" as indicated in the screenshot below. Upon clicking it will automatically alter the default behavior.
Then, once the page has been refreshed you should see the menu titled "Enable Automatic Credit Usage". Upon clicking it again you'll enable back the automatic usage of your credit to clear your unpaid invoices.