Amazon offers up to a $20 Amazon Unlimited Music Credit for free when you sign up for select Amazon Music Unlimited Plans with coupon code “AMAZONMUSIC”. (Individual plans receive a $10 credit, family plans receive a $20 credit.)
Plans start at $7.99/month and include your first month for free. Amazon Unlimited Music features “tens of millions” of songs that can be streamed across all of your devices.
Note that the gift card will appear after your trial month is up; if you cancel before then, you won’t receive the credit. However, you are still free to terminate your plan at any time beyond that.