Yes,that is correct. They do deliver mail and other items the captain and crew may need. The boat that delivers the mail and some times river captains for foreign vessels is the Prescott. A year or two back the Prescott capsized killing some of the crew. This boat is the oldest mail boat in the country I believe.
Once along side these big freighters, the captain can actually leave the helm as the wake holds them snug to the side. Obviously, they have to hold speed to keep pace. One of my friends said she had the chance to ride the Prescott, and delivered a goat! I didn't ask for details.
Jimmyboy