Short Sea Shipping Act of 2019 This bill exempts certain commercial cargo from the harbor maintenance excise tax. The exemption is for commercial cargo, other than bulk cargo, that (1) is loaded at a port in the United States mainland (i.e., the continental United States not including Alaska) and is unloaded at another port in the U.S. mainland after transport solely by coastal route or river, or a port in Canada located in the Great Lakes St. Lawrence Seaway System; or (2) is loaded at a port in Canada located in the Great Lakes St.. Lawrence Seaway System and is unloaded at a port in the U.S. mainland.