NOTE: THE OP CHANGED THE QUESTION AFTER REPLIES WERE POSTED. SEE BELOW.
In the original question he said he was a volunteer, and he said that donors send money to him through the 501(c)(3) organization. The following is my reply to the original question.
You're not a volunteer if you are getting paid. It appears that the organization is paying you for services that you perform for it. If you are not an employee of the organization, then you are self-employed. The legality of that arrangement is debatable, and might be affected by the fact that you are working in a foreign country.
The bigger problem is on the other side of the arrangement. The donors cannot claim a tax deduction for a donation that is designated for a specific person. If the organization is representing to the donors that their donations to you are tax deductible, that is clearly illegal.
