You can ask for extra details such as eyes, pawpads, headshot, etc. Each extra ranges from 5 to 20€ depending on its complexity

Having no previous reference of your character will increase the price by 50% (including extras)

Remember that ref sheets are not regular artpieces. They are made to be used by other artists, so they need to be clear and efficient, not fancy. Everything should be kept as simple as possible so when asking for extras or when chosing the infos to be added on the ref, always ask yourself “Is this essential to a good understanding of my character and its design?”