I just posted the same information on another thread - but you may not see it, so here it is.
We rented from National this past December. They pay cab fare for you to get from the boat to their office. When we disembarked, we told our baggage handler we were renting from National. He took us to a tented area in the pariking lot, near where the Avis van was loading. He asked the CM to call for a yellow cab for us.
The cab arrived in a few minutes and took us to the National office, about 10 minutes away. The office was not crowded, and within 5 or 10 minutes we had our car. We only had to tell the National person that we took a cab, and he subtracted the $11 from our rental charge.
We have relatives who rented from Avis because it was closer and had free shuttle, but the free cab is quicker and so much more comfortable.
Suzanne