I've got one 35% coupon, a bunch of 30% and a bunch of lesser coupons. Some of these did not work for me - YMMV. (Frustration? I haz.)
If you have success with any of the codes, please let me know. I am trying to figure out the pattern. (Other than "this is all part of Kobo's massive psych experiment.)
35%
NH957Y UK
30%
25% or less