Amazon Original Stories | Short Reads (Free with Prime)

Each story seems to be priced at $1 to $2 or around $5 for a collection for non-prime subscribers.

Each ebook is also bundled with an audio book (for prime subscribers at no extra cost).

Kindle (.mobi) format only so best accessed with a Kindle device, but also accessible via Kindle Web, Android, iOS apps as well as other options if you're savvy enough to figure that out.

Looks like the free offer is for US prime subscribers only.

There is a email newsletter for notifications of new collections.

Other newsletter options look useful too.