I got used to "Send to Kindle" service to populate my Kindle with e-books from external sources. I can see the files in "Manage Your Content and Devices → Docs" section in my Amazon account.
Today (Mar 27, 2015) I received an e-mail from Amazon that states
Your 5 GB plan is no longer available and has been replaced with a free 3-month trial of one of the Unlimited plans. Access to your existing files has not changed. You can continue to download and view your content. In order to upload new files you will need to pick one of the free 3-month trials.
I cannot find any information on whether I can continue to use the storage for free or I need to apply for some unlimited plan.