I finally bit the bullet on this. I was hesitant at first because I thought it may have been a mistake that they included sacks with free shipping and I didn't want to take advantage of them. But if it's been going on for months, it's clearly a deal that they're aware of. I have 4 sacks in my cart, and when I look at the shipping options it's like $160 for UPS Ground if I don't select Free Shipping. I'm not sure how they make money on this deal, but I suppose they do somehow?
I did notice that the 15% code didn't work for Proximity base malt, but I'm ordering two sacks of Briess (2-row and Pilsner) and two sacks of Swaen (Pilsner and Lager). I've never tried Swaen before.
One other quirk I just noticed - when I had the 55 lb Proximity sack in my cart, non-free shipping showed up in the $130 range. When I deleted that and put in 50 lb Briess 2-row, it went up to $160 for shipping. I'm not sure why it would cost more to ship a lighter bag?