The phrase "one's voluntary contributions" refers to the things that someone willingly offers or donates without being forced or obligated to do so. Full definition
You can make voluntary contributions in multiples of $ 25. (opm.gov)
Some union leaders have expressed fear that being forced to rely on voluntary contributions for their political activities would dramatically reduce their power and influence. (protectteacherrights.org)
Schools are not allowed to charge for education in school hours, but no law prevents them from asking for voluntary contributions for the benefit of the school or any school activities. (bbc.co.uk)