Best way from car rental place to docks and back to Houston when cruise returns

[Gary Patrick Higgins]

We are planning a trip on the Carnival Triumph between Christmas 2015 and New Year's 2016. We live in West Texas so we are going to rent a car, drive down to Galvestion, and then fly back. What is the best/cheapest way to get from car rental place (hopefully we can drop off in Galveston) to the docks....and then when we return, we want to find the same best/cheapest way to Houston to fly back to our West Texas home. Apparently there is nothing that flies out of Galveston. Anyone have any good experience/suggestions? Gary & Schamane