USPS, as well as the local, state and federal agencies in the storm damaged areas, are still in search and rescue mode. There are employees in Beaumont postal facilities that cannot be reached. Other than search and rescue, the instructions to USPS employees is to shelter in place. Damage assessment hasn't even begun in much of the Houston area.
Mail processing is taking place in Austin and San Antonio. Mail that can be transported for delivery is going out, but mail for the affected area is being staged, or held in Dallas.