Updated: Jan 22, 2021
For the ultimate cushy ride, consider upgrading to a suspension seatpost. This one, the SR Suntour SP12 NCX, is also sold on Rad's site and is widely recommended by Rad Riders, who say that even the stock Rad seat is pretty darn comfy with this seatpost. But for the "Ultimate Comfort Combo," we'd recommend a saddle upgrade as well... After all, for under $50 you can pick up a Cloud-9 and be riding smoooooth.
For the seatpost, make sure to purchase the 27.2 mm for your Rad, which comes in 2 lengths, 350mm and 400mm. In our house, I'm 6'5" and went for the 400mm for my Rover, but my wife is 5'4" so we got her a 350mm for her City ST.
Below is a great video from Ebike Escape that shows this "Ultimate Comfort Combo," the SP12 post with a Cloud-9 seat installed on a Rad Wagon. In the video, they call it a "game-changer," and we'd have to agree!
Here is the latest pricing/availability for each:
SR Suntour SP12 NCX Seatpost: AMAZON
Sunlite Cloud-9 Saddle: AMAZON