Here's the main link for men's underwear. [duluthtrading.com]
Click the free shipping offer for $75+ in the upper right corner of the page.
I searched and didn't see any duplicates. $16-$17 is usual sale price for Buck Naked, and I don't think I've seen Armachillo this cheap ever - last sale I saw was $22.00.
For the sake of completeness, edited to add:
Free Range is at $14
Bullpen is at $20
Women's side also appears to have sales, but I have no idea how good the prices are.