We rely entirely on voluntary contribution.

我们全靠自愿捐赠。

《牛津词典》

Most Americans advocate the spirit of social service and voluntary contribution.

大多数美国人提倡社会服务和志愿贡献的精神。

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It described the target as a voluntary action, and predicted it would make a "major contribution" to global efforts to deal with climate change.

VOA: standard.2009.11.26

They said they would not be following Starbucks' voluntary contribution lead.

BBC: Boris: Don't sneer at Starbucks over tax payment

It is also expected to feature a voluntary contribution from private investors, who will be invited to buy up new Greek bonds as old ones mature.

BBC: Greece: Eurozone ministers delay decision on vital loan

This review was a voluntary contribution to the BBC website.

BBC: Review: Eastern Angles' Gills Around The Green