Rule reminder
Attached-to-infinitive position: drop the final -e of the infinitive, then attach the double pronoun as one word. Example: 'Posso dartelo' (I can give it to you). Combined forms attach directly: dare + te + lo = dartelo; mandare + glie + la = mandargliela.
Quale è corretto per 'She wants to send it to him' (la lettera) — attached to infinitive?