Encargo

Encargo
 

Es un encargo.
Inglés?
Context.
Un encargo que mi hermana me ha hecho, comprarle un sweater en América.
Is there an equivalent for that word in English?

There isn't a noun for an informal arrangement.

If you send somebody to get something, you send them on an errand. But in this case, I guess you are going anyway to the USA, not going especially on an errand for your sister.

A more formal word is commission. It is used for a formal arrangement, but used amusingly here:
My sister commissioned me to buy her a sweater ....
My sister asked me .....
My sister orderded me...
My sister got me to promise to buy her....
My sister arranged for me to....

Entonces el suéter que tu hermano que te encargó comprar tu hermana
es una encomienda. ¿Estoy en lo cierto?

:):):)

Podría ser, si se toma como siendo encomendada con la tarea. Encomienda también significa
"parcel"