The U.S. Postal Service is asking for a temporary waiver from President Joe Biden’s vaccine mandate, warning it could result in “substantial harm” to the beleaguered supply chain.

In a letter to the Occupational Safety and Health Administration dated Tuesday, Deputy Postmaster General Douglas Tulino argued that imposition of the vaccine-or-testing edict just days from now would likely “result in the loss of many employees — either by employees leaving or being disciplined — particularly given the higher level of temporary employees at this time of year.”