I hope this was not too tangled

On the contrary, it was quite untangling, Marianna.

We are fortunate to have someone with your depth in Spanish to clarify this for us.

I am guilty of making the connection between "volunteerism" and "Buena Voluntad".

I was projecting my sense of the meaning of "Buena Voluntad", not a literal translation - the 'sense' being that "volunteers" typically do things "pro bono", for the public good, without remuneration.

This is the sense in which I understood the name "Buena Voluntad", the Barcelona organization which undertakes charitable works in the "4th world".

re "goodwill" in English is a synonym for "charity", i.e. an organisation or group that works to help the poor or otherwise deprived.

"Goodwill" is not really a synonym for "charity" [i.e. a charitable organization] in english, Marianna, although some charities use the word in their name, as Elizabeth Creith has noted just above.

"Benevolent society" is a better term for a charitable organization in english, I suggest.

In fact, Merriam-Webster cites "benevolent society" as an example of a group organized for the purpose of doing good:

Main Entry: be·nev·o·lent

Etymology: Middle English, from Latin benevolent-, benevolens, from bene + volent-, volens, present participle of velle to wish -- more at WILL
1 a : marked by or disposed to doing good <a benevolent donor> b : organized for the purpose of doing good <a benevolent society>
2 : marked by or suggestive of goodwill <benevolent smiles>