So I just got the update notification for Scrivener 3. The question says: “Would you like to download it now?” But there is no such option.
I think there should be a Buy and Download Now option… there is only a “Learn More” option.
We use the Sparkle framework for automatic updates, which is what most developers use. The text at the top of the panel and the buttons available are determined by that - you can either provide a “Download” button or a “More Info” button, but not both. (I may report this as an issue to them, come to think of it, given that the “Would you like to download it now?” text doesn’t tally with the “More Info” button). We opted for the “More Info” button rather than a download link because we didn’t want Scrivener 2 users accidentally downloading and overwriting Scrivener 2 with Scrivener 3, which is a paid update. (We made that mistake with the Scrivener 1 > Scrivener 2 update - we mistakenly assumed people would read the text in the update window about it being a paid update. 
)
All the best,
Keith