Based on what I've heard about Costco, they're literally the only company that I could read this letter and think "yeah, they're probably genuine about their sentiment".
Unionizing is ALWAYS a good idea, just in case, but as far as companies go, Costco has always gone out of its way to make sure their people are taken care of.
The above is a study from the University of Texas. The conclusion seems to be that yes...it's still retail, with all the usual crap that comes with it, but the company actually tries to mitigate it as much as possible.
For real. The kicker for me is that I recognize almost every worker at my local Costco. They aren't just new faces every week. I worked retail for years and I've never seen a retail place retain so many people for so long.
When I was a low-level retail manager my perspective was that my staff was there to take care of the customers, my bosses were there to take care of the store, and my job was to take care of my staff.