It took me a minute to figure out what you were saying here, i.e. "shop till people" = "cashiers" (people who work tills in shops). "Till" in that sense is rare this side of the Pond ... at first I thought you were saying "... shop until people ...". :-$
Again with your main point, "whether you wouldn't like any cash back" vs. "whether you would like any cashback". Again, nothing to do with my first impression -- "if you wouldn't like" vs. "if you would like" ??? ... oh, no, you're talking about the difference between "cash back" (phrase) vs. "cashback" (compound noun).
* You may be right, though I'm not sure it's defeating/defusing the marketing executives' efforts. Even though we know it's bullshit/bollocks, somehow "cashback" sounds like a different (better!) sort of thing, as though it were more than just giving you some "cash back".
Garden pathing so hard ...