They seem to have an union at least here in Spain, and maybe in other countries as well. Last month, workers at a local Amazon ware house in Madrid did a 2-day strike, which was pushed by a national workers union as a complain on the working conditions [1]. I've been out of the loop on news so I don't know how things are going since then.
Boycotting usually doesn't have the same effect as a 2-days strike.