I think the cannonical "efficient" charity would be incenticide treated bug nets. Against Malaria Foundation distributes them to entire villages, but the largest effect is for children 5 and under, for who catching malaria is much more likely to be fatal.

Grain has some potential negative effects, squashing local food industry, etc, but it's not like domestic bug net companies and a key part of the economy in the same way farming is.

I dunno, I feel like stopping kids from dying is better to focus on first compared to letter writing. But I'm a bit of a Utilitarian, and an aspie, so perhaps I undervalue the human element.