> pay for events
What events? If it's to discuss a certain issue, that can be virtual and almost free. If it's a social event, meh, there are enough of those already and people can self-organize them. Hell for $500/month I could organize social events where everyone goes to Michelin 3-star restaurants every month and rants over gourmet dinners about their bosses.
> pay for mailers and other communications infrastructure
Use e-mail. I don't even check my snail mail box anyway unless someone tells me to expect something by e-mail, and even then when my snail mail box gets too full I usually just dump it all in the recycle bin, so it's not an effective way for a union to communicate with me.
> and of course pay for professionals like labor lawyers.
What if they just split the lawyer fees evenly?
Assuming an experienced lawyer charges $1000/hr and spends 100 hours on a case, and there are 1 million people in the union, that amounts to about $0.10/person/case, a far cry from the $500/month they seem to be charging. Even if my numbers are off by a factor of 100 it would be only $10/person/case, and if the union won the case Amazon would probably have to pay the legal fees anyway.
Although this may sound a bit naive I feel like $500/person/month sounds like there is some massive inefficiency in use of resources.