URComped’s cruise offers are for double occupancy cabin categories, so if you qualify for a comp cabin and you are traveling solo, there is no surcharge. You would still need to pay the full port fees and taxes for the double occupancy cabin since these are required to secure it.
If you think you might add a guest later, it is recommended to include a name for the second guest when you book in case the ship reaches its mandated limit. If you plan to bring another adult guest later on, you can choose anyone and swap the name before you sail.