Apple is removing support for legacy extensions in Safari 13. They’ve been encouraging developers to build Safari App Extensions instead. We’ve now done that for Push to Kindle and published it on the MacOS app store.
You can download it here: Push to Kindle for Safari
Due to Apple’s naming requirements, we weren’t able to list it as Push to Kindle and had to name it Push for Kindle instead for the app store. And because Apple charges developers to develop for its platform, we’re selling the extension now, which we didn’t before. (It’s still free for Firefox and Chrome.) If you’re a user of our previous (free) Safari extension, you can email us at firstname.lastname@example.org and ask for a code to download the new one.
Our old extension is still up on the Safari extensions site, and should continue to work in Safari versions up to and including Safari 12.