If you booked the cruise through a
travel agent, your agent will need to arrange transportation. I tried booking post-cruise transportation myself, and as soon as the
DCL cast member saw the travel agent on my account, she told me I had to book through the agent.
In general, anything involving a change or immediate payment on the reservation needs to go through a travel agent if you used one. That includes changing rooms, changing cruises, pre- and post-cruise transportation, and adding or removing guests. Anything that occurs onboard and is charged to your room account, including excursions, spa treatments, and dining, can be done on your own. You can also purchase onboard gifts (decorations, flowers, etc.) yourself, since those don't affect your reservation.
If you want to pre-pay gratuities, that's done through a travel agent. However, if you want to buy yourself onboard credit that can be applied to gratuities (or anything else on your account), I believe that can be done through onboard gifts.
