If I volunteer for a non for profit agency and is stated on the letter saying I am not an employee but the end client pays me for service render. I don't get a paycheck or t4 slip, am I considered an employee? Also I am person with a mental disability.

Whether you are an employee is not determined by the agreement you signed, but instead the reality of the situation. Therefore, the law will look at the actual relationship (i.e. your work schedule, reporting structure, duties, etc.) to determine whether you're an employee.