Just to put an alternative on the table. For a direct translation, I think Antony's is excellent. This is a different approach, obvious much freer as a translation but the kind of thing charities often say to thank donors. "Please give generously" / "Thank you for giving so generously". I think it would go well in the context of an expression of thanks: not so much analysing their motives but praising their unselfish generosity. "Unselfishly" would be a bit closer to "desinteresada", but "generously" is what I think would be said.

Spanish term (edited): de forma totalmente desinteresada

Another take on it from a different angle...

:)

There are a number of ways to translate this. Other offered are equally valid

Another alternative.