The easily forgotten thing here is that they only need enough scabs to deliver what's in the pipeline. If UPS goes on strike, the customers will ship their stuff with someone else. And since UPS is one of the only unionized package carriers, the freight will not be refused by the likes of FedEx.
I'm not suggesting that there's enough scabs to replace UPS. I'm saying there are plenty of other companies that are not unionized who could pick up the slack. It wasn't the end of the world for customers when CF collapsed, was it? Several thousand trucks and trailers off the road one day and almost nobody noticed because everyone else stepped in to fill the void.
It may be a driver's market, and UPS sure as hell isn't going to shut down, but the world will still go around if UPS goes on strike and the only thing it'll serve to do is cost UPS customers, giving them an excuse to lay people off. And that's before we get into how Hoffa and Co. are in UPS' pocket.