About 260,000 UPS workers are represented by the Teamsters, and their current contract with the company expires on July 31.
The members' vote "gives the negotiating committees bargaining leverage this week and during subsequent negotiations for the national contract and the supplements," Denis Taylor, a Teamsters director and co-chairman of the bargaining committee said in a statement reported by Marketwatch.
A UPS spokeswoman told Marketwatch the company is confident it can reach agreement with its workers.
The bargaining issues involve pay and work schedules, including expanded service during weekends. UPS started making regular Saturday deliveries last year. The union said the company has made proposals for expanded weekend services, which may include Sunday deliveries.
UPS shares rose 21 cents, or 0.18%, to $117.02 in Wednesday trading on the New York Stock Exchange.