Something that is ancillary is additional or supplemental. The ancillary costs of something are those “extra” expenditures beyond the sticker price of a product or service. This sense of ancillary is closest to extra when the latter is used to refer to things that are additional or go beyond what is expected. But ancillary suggests a more defined relationship than extra, usually indicating a subordinate or subsidiary position. Used in a medical context, ancillary feels quite far from extra: in this context ancillary describes services or staff that provide essential support, as in a hospital. So remember, context is key!

The noun pinnacle is used to talk about the highest or culminating point of something, most commonly the time in one’s career when they are most successful or a specific achievement that caps one's career. In architecture, a pinnacle is a small, upright structure rising above the roof of a building or capping a tower. In nature, a pinnacle is a lofty peak of a mountain or a pointed, towering part of a rock formation. The theme of capping, crowning, or being positioned atop something is what threads these senses together. The more general word peak is the appropriate choice for talking about maximum prices (oil prices reached their peak) or maximum volume (traffic was at its peak).