United States Postal Service Stabilization Act of 2013 - Amends provisions concerning the Federal Employees' Retirement System (FERS), with respect to employees of the U.S. Postal Service (USPS), to: (1) provide a new formula for calculating employee pension contributions using the normal-cost percentage method multiplied by the aggregate amount of basic pay payable to employees of USPS and the Postal Regulatory Commission, and (2) provide for the use of excess FERS contributions to pay USPS debt obligations and make pension contributions.