Imho there are different aspects to this: are they binge buying useless stuff on Amazon constantly, feeding into mindless consumerism? are there more ethical alternatives available?

If someone uses Amazon for something they can't find anywhere else, or that the alternative costs so much more they can't afford it, then I understand. If they are just a mindless consumer and they do nothing to change that, they lose my respect.

At the same time, if that is a reason you are considering to break up with someone, imo it tells your relationship is not very solid to start with.