I just ordered two K-Rad 20 x 3.3" tires and two Kenda 20 x 4.0" tubes and the total shipping was $34 (to NY) so they must be giving some kind of multiple unit order quantity shipping discount. They broke the shipment down into two parts, tires & tubes coming on separate tracking numbers.
I'm still ticked off about their "accessories ship free with bike orders" promotion though. When I ordered my bike, they were out of stock on almost every accessory I wanted and you couldn't add out of stock items to the initial order to qualify. So, I've now shelled out about $60 in shipping charges on the things that have become available since (front & rear racks & large basket).
Another thing I've noticed about Rad is their back in stock notification emails that you can sign up for can be a bit late. I noticed the tubes were back in stock the other day when I went to order tires (after trashing one), and the back in stock email I had requested showed up a couple of days later. Same thing with the black RadMini Rear Racks. Even though I signed up, I've never received a notification that they were back in stock but I did see them listed one day, grabbed one, and they were out of stock again by the end of the day (and remain so). So, if you really want something, it just might pay to check regularly.