At $8 per month, Amazon Music Unlimited was already more affordable than Spotify Premium or Apple Music, but Amazon’s sweetening the pot for Prime members with a limited time $10 credit when you sign up for a new account and use promo code SUMMERMUSIC.
You’ll still get your one month free trial when you start the service, but the first $10 you spend on the service will be automatically refunded thereafter. That means you’d over two months of free music for an individual account, or a deeply discounted second month of the family plan. Just note that this promotion is only for new Amazon Music Unlimited subscribers.