How strange - I went to the site, had to click 'English', clicked to the Mac App Store link (not USA) which then transferred me to my MAS. To my surprise I appear to have already bought it (but didn't have it installed), although I have no recollection of having done so, and cannot find a record for purchase. So I downloaded it.

Here's the US MAS link ($9.99):

https://apps.apple.com/us/app/wonderpen/id1173437104?mt=12

The app has apparently been around since 2016.