In principal, all businesses ask for cash donations for their goods or services, as can you. Do not be confused, however, the contributions, made before or after the service is rendered, is revenue to you, and must be accounted for and taxes paid on.
I would strongly suggest legal and accounting counsel given the nature of your question.
To the PROSPECTIVE client, please call myself or another attorney for your choice with more detaiils and an appointment. My PRELIMINARY answer to your question(s) is for general purposes and based upon what little information you have conveyed. It is based on such limited information that the general answer should never be relied as a reason for your action or inaction. My response does NOT establish an attorney-client relationship and such may only be established by mutual agreement, and the signing of a written retainer agreement, which will generally require payment for our services, as this is what we do for a living and, just like you, we must get paid for our work..
I agree with the other anwers. Of course you can receive your compensation in whatever form you choose, but it is still revenue to you and/or the LLC. I believe the question you are asking is really a tax question, and you should consult witha tax advisor.