Amazon gives non-Prime members free shipping at $35 or more of eligible items. Instead of simply letting users get the product with free shipping, they’ve added a discount that prices it exactly one cent below the $35 limit, while only subsidizing the price with $3.38, which is about half of what they’ll then charge you for shipping.
I’ve actually added very low cost items to my cart, made sure free returns from Amazon only, and returned it. Two can play that game Amazon!
If you return ‘too many’ items they may suspend or even ban your account. I’m not sure what the threshold is.