I am also using Safari on Mac Mojave. So far, although different, I have had no issues with downloads. 1) I am logged in, 2) Click on Add to Cart, 3) Click on Get my Downloads, 4) Click on Send Me The Link. At this point a page called Purchase Confirmation with the link appears. The link is also sent to me in email. From this page click on the link and you get the We Transfer Page.
Hope this helps.