When you add funds to the Credit Balance, your invoices are set to be paid as soon as they are created. Note that the invoices created before adding funds to the Credit Balance are unpaid, and you must pay them manually.
Learn how to add funds using this guide.
Log in to the Portal and go to Profile -> Settings and Billing -> Payment Methods.
Click the Payment Methods tab and then click Add Credit.
Select the Payment Method and enter the Payment amount you want to add to your Credit Balance, then click Add Credit. Note that the minimum amount is $50.
Pay the invoice to add money to your Credit Balance successfully.