WFP received $4.8 Billion in 2015 in voluntary contributions. Of that, $4.7 billion was provided by governments. Private donors contributed $98.3 Million, about 2% of the annual revenue (Source: WFPUSA Annual Report 2015). All government support is on an entirely voluntary basis. The organization receives no dues or UN assessed contributions.
Similarly, UNHCR is funded almost entirely by voluntary contributions, with only 6% from the private sector, including foundations, corporations and the public (Source: UNHCR – Figures at a Glance)