Amazon is throwing some developers a bone with a promotional credit on AWS for Alexa skills to help cover any incurred usage charges. Most of them use the free tier, but some do end up exceeding those limits and incur charges. Any that do can apply for a $100 promotional credit to help cover those costs.
This effectively makes it free to build and host Alexa skills. But wait, it gets better. If a developer continues to get dinged with AWS charges that exceed the initial promo credit, he or she will be eligible for monthly recurring credits to continue to host on AWS. That $100 is good for a lot of Amazon's services like EC2, S3, DynamoDB, and CloudFront.
Encouraging developers to host on your platform by offering promo credits is a pretty good idea. If you are in the targeted demographic, hit up the source link to get some more information or apply for the credit. And if you're looking to get started on developing an Alexa skill, it looks like now is a good time.