I was recently contacted by a popular YouTuber wanting to know more about how I work and how I charge for my work before we can talk about designing... and I just realized that I have no idea what to tell them as it's my first time doing freelance (instead of designing/selling my own products). How much should I charge for a phone case design (for example), assuming it will be sold to, potentially, hundreds? And should it be a flat rate or should I actually ask for a small percentage of each sale? Either one is fine for me, I do this stuff as a hobby anyway, as in it's not my actual job. Thanks for the help in advance. Sorry for being clueless, I just need advice as I don't want to mess it up.