If it is helpful - I think the next strike deadline is August 25th, plenty of time to know and figure out alternatives before send outs - and before sign ups end. But the union has the option of either going on strike, or extending the talks. The union has to issue 72 hours notice if they are going to strike or not. I guess if worse comes to worse, we Canadians can drop out (wahhhhhhhh!) - or have alternative forms of shipping - like UPS, or some other small carrier.
Or it could be a tempest in a teapot. If there is a strike, I can see the government stepping in within a few days to a week, and legislating workers back to work, and possibly forcing arbitration on both parties.
With those dates maybe it would be over before Sept 12. I imagine there would be a backlog of mail but it would get out.