I went to work for very small non-profit. I initially thought it would be a regular job but quickly found out I would be considered a independent contractor. I served in that capacity for nearly a year and a half and then was informed I would be moved "regular part-time" status and the NP would begin taking taxes, etc out. No problem. I finally decided, to resign in the last month. I thought it was odd that I was asked to provide a letter of resignation seeing that I never signed any type of employment contract/agreement. When I gave my resignation, I shared what my last day would be (I gave them a month's notice!). In addition I offered my services as an independent contractor on a limited basis. Should this letter include anything outside of my last day and that I will be available on an as needed basis?
They asked for a letter of resignation so that they can keep something on file, in writing that states you've voluntarily terminated your employment there and when your last day is. It's nothing nefarious.
Just give them your final day, thank them for the experience, and that's all you need to do. Offering services as an independent contractor should probably be done as a separate communication.
Should this letter include anything outside of my last day and that I will be available on an as needed basis?
Nothing more is necessary.
Often people include their general "thanks" in such a letter (so as to pave the way to future good relations and recommendations, etc). But that isn't necessary.