the letter at the school after not finding Harriet there, which I doubt, I think he had planned to propose by letter.
>had left a little parcel for her from one of his sisters, and gone away;
The letter was in the parcel made at home, with the other things.
I think he hadn't planned to say at all. He would have delivered the parcel in person to Harriet, exchanged a few niceties and left. Harriet would have opened the parcel later...followed by the same situation.