The first thing you'll need to do is to make sure the credit is activated so the system will apply it to your next billing payments. You may view/activate your credit on the Billing page:
Steps to activate your credit on the "Billing page"
Click on Fulfillment located at the top-left of your dashboard
Click on Billing located on the top right of the screen
On the next screen, you will see a section titled Credits that may be activated
You will first need to activate the desired credit by switching the OFF button to ON
A pop-up will appear, please read and then click on Apply Credit
Once the credit is activated, the credit will automatically apply to the next order you place in the store or the next bill date for a subscription you may already have.