If one looks at the whole economy, some prices might be very flexible and others rigid. This will lead to the aggregate price level (which we can think of as an average of the individual prices) becoming "sluggish" or "sticky" in the sense that it does not respond to macroeconomic shocks as much as it would if all prices were flexible. The same idea can apply to nominal wages.

https://en.wikipedia.org/wiki/Nominal_rigidity

If Amazon were to open a subsidiary for lower priced delivery services it might be able to maintain its workforce branding and add lower rungs to the wage ladder for the purpose of labor supply development. It might be politically inconvenient for a wealthy company like Amazon to have an acknowledged wage tiering system even if it makes sense on different levels. For example, Amazon probably cannot publicly hire people without workforce documentation--an unacknowledged arms-length subsidiary would be able to provide this service.