Legal Question in Business Law in California

Re: agreement to provide monies to non-profit agencies

Sounds as though what you want resembles a royalty agreement. As such, the 'letter' you want is a form of contract. An experienced business attorney should be able to interview you to determine exactly what's required, find a suitable model form agreement and modify it to fit your particular situation.

If the person remitting the money is seeking a tax deduction, be aware that not all nonprofit corporations are also IRS qualified so that donations to them result in charitable deductions.

Otherwise, my hunch is that what you need is a species of contract, and that you will need professional assistance in drafting it, especially if you will be seeking the nonprofits' cooperation or acquiescence in anything.