No, they used to let you pay online but they took that away a few years ago. Currently you pay as you go in, they have machines you put your credit card in. They usually have an attendant there to help. We heard rumors on our last cruise that they eventually may switch to pay as you exit which would then capture a fee from all the back to back cruisers who currently get the parking for the 2nd trip at no extra cost.
The parking garage is a decent size you should be fine. We don't park in the preferred lot, I'd rather have my car in the shade and out of the weather for loading and unloading.