The last time UPS workers walked of the job more than two decades ago, it crippled the shipping company. An emboldened union is threatening to do the same and this time the disruption would be far greater. The 24 million packages UPS ships on an average day amounts to about a quarter of all U.S. parcel volume, according to the global shipping and logistics firm Pitney Bowes. UPS says they deliver the equivalent of about 6% of nation’s gross domestic product. The Teamsters, representing about 350,000 UPS workers, say they'll strike if there's no deal by the time the current contract expires July 31.