I agree with sending it right before you leave. Give her time to let it sink in.
Do you think handwritten would be more personal? If so, write it out, then leave it on the counter when you go. Otherwise, e-mail it to her, and maybe a verbal, I sent you an e-mail.
I don't know handwritten ... or email.. i think handwritten is more personal but email is there always, unless she deletes it.
