Clerk-to-clerk variation on phone coupon use has been my experience as well. One checkout clerk at my local store (a woman in her early 20's) asks to hold the phone and critically examines each coupon. Instead, I get in the line for one of the two older clerks (in their 50's) and zoom way in on the coupon codes for them. I say, "I can never read the email coupons when I print them out at home, so I zoomed in on the code on my phone so it's easier to read, I hope that's OK. " And they are all smiles as they type in the codes for me. I open a separate window and zoom in for each coupon before I get in line so I can just switch screens quickly - it works great and they can't even tell if it's a bad scan.
And some folks just don't care at all - I stopped at a HF on my way through CT last year and the clerk flat out told me, "Buddy, you're going to have to read me your coupon codes from your phone because there's no way I'm going to be able to see those numbers."