Merchant Marine Act Amendments of 1986 - Amends the Merchant Marine Act, 1936 to require the Postal Service, when practicable, to use U.S. registered vessels for the transportation of mail by ocean carrier. Directs the Postal Service to award mail transportation contracts on the basis of competitive bidding. Revises provisions concerning the payment of the operating-differential subsidy. Permits a vessel receiving an operating-differential subsidy to operate on an essential trade route unless the Secretary of Transportation makes certain adverse determinations concerning such operation. Directs the Secretary to contract for the payment of an operating-differential subsidy on a vessel constructed or reconstructed in a foreign shipyard after January 1, 1980, and registered under U.S. laws if such vessel is suitable for national defense purposes and is 5,000 deadweight tons or greater. Requires such subsidy to be paid for the first five years after the vessel is registered under U.S. laws. Authorizes the Secretary to allow an operator receiving or applying for an operating-differential subsidy to own, charter, or operate a vessel not receiving such a subsidy, or a foreign vessel. Prohibits any operator receiving such a subsidy from operating a vessel engaged in the domestic, intercoastal, or coastwise service. Directs the owner or operator of a vessel receiving an operating-differential subsidy based on a return voyage in ballast (empty) to reimburse the Government an appropriate amount when the vessel: (1) carries cargo on any segment of the return voyage; (2) is scrapped in a foreign country; or (3) is sold to a foreign person and title is transferred prior to the vessel's return to the United States. Requires the owner or operator to notify the Secretary if any such event takes place. Requires the Secretary to maintain a list of vessels that may be scrapped in a foreign country and to provide such list on a monthly basis to all agencies using privately-owned U.S. flag vessels to transport preference cargoes.