The origin of the modern union movement (as opposed to old guilds) has a lot to do with the demand for 8 hour days, weekends, and regulation of child labour -- not exclusively issues of renumeration.
Unfortunately part of the post-WWII settlement between labour and capital in the US and Canada had to do with giving up a lot of the shop floor power that unions had won during the 30s, the war, and immediately after it -- in exchange for compensation and job security. The AFL-CIO took a far more moderate political line, and clamped down on socialist and communist agitators within its organization. And when the right wing and austerity forces became more ascendant in the late 70s and into the 80s they were left with no real political tools to fight it with. Unionization, and compensation, declined. And unionization got its reputation as a bureaucratic and often corrupt practice as it failed to do much to help its workers.
Kroger: $8.93 avg. https://www.payscale.com/research/US/Job=Cashier/Hourly_Rate...
I had no idea until now, but with this in mind, I imagine unionization at Amazon will be an incredibly hard sell that is unlikely to happen unless that 42% gap shrinks significantly.
I know that I've seen union badges on employee uniforms in the public sector. Maybe not at the uniform level, but even a plaque or playing it up in advertising now that buy local is a massive thing.
(Of course, half the US population seems to reflexively shudder at the word "union" these days, so how successful this would be is... debatable.)
Whole Foods seems particularly difficult, though. In my area, there aren't a whole lot of places that occupy "that" corner of the market. (Organic, sure, but the main thing I'm looking for is clearly labeled local produce, bulk bins, and some harder-to-find ingredients such as vital wheat gluten.) The only ones I can get to by public transit in less than 60 minutes are Whole Foods, and a grocery co-op. And I'm inclined to guess that, however crappy Whole Foods may be about this sort of thing, their compensation is still way better than the "15% off your groceries as a thanks for volunteering, and the satisfaction of being more involved in our community" that the co-op is offering to the people who work their registers.
Also an argument in favor of sectoral bargaining.
Half?