They said I don't need a business license, I can work under theirs.
Take that with a grain of salt. In my experience, as limited as it may be, even a sole proprietor needs at a minimum, an "occupational license" to operate as an unregistered sole proprietor. Typically, (and in my limited experience with this) the occupational license or it's equivalent is issued by the local county or township the business will operate in. So you need to check with your local county courthouse or township tax collector's office.
I can't state this for a fact, but I doubt you can "work under theirs" unless you are a W-2 employee of whoever "theirs" is.
Depending on your state/county/township, I wouldn't expect an occupational license (or it's equivalent) to cost much at all. For my computer technology business I only pay $22 a year and that's all I've paid each year since I started in 2005.
My spouse works for a local dentist, and his occupational license is $225 a year. An occupational license for anyone in the medical field is the most costly one in my county too, at $255 a year. That's not even a drop in the bucket for a doctor with a private practice in my locale.
