During the checkout process, you will be given the option to enter a valid discount code or gift card number.

To do this, you will click where it says Add a Gift Card or Promotional Code on the Complete Your Purchase page:

You will then input the code into the box and click Apply:

Please note: When you add a gift card code at checkout and click Apply, the balance on the gift card will be applied to your gift card balance. You can then use this payment method when completing your purchase. See the below screenshot for an example:

Below is the a video that explains how to use the Split Payment option with an account balance and a credit card. 
 

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