On February 26th, Kindle customers will lose the ability to download eBook purchases directly to their PC. If you want to switch to a rival eReader brand in the future, I suggest that you use the soon-to-be discontinued “Download and Transfer via USB” feature to archive your Kindle library.
Addendum: that specific site is dog shit. Imagine thinking you just bought an ebook but instead you bought a lease to some DRM shit that only works on their app.
EPUB or GTFO.