The credit should automatically be reduced at checkout. If this doesn’t happen, I suggest contacting Amazon customer service. They are fabulous.
My credit didn’t show up (when I bough Magic School Bus, actually!), so I ‘chatted’ with them and they instantly gave me a $5 credit in my account.
You do need to purchase something $25 or over, which is sold thru Amazon.com – not one of their third party providers.