Registered mail is safest, you can also get a return receipt for that. That allows you to have proof of the delivery. In theory only the addressee can sign for Registered mail ( I was a Letter Carrier for 35 years and as I said above too many poorly trained non career employees now) so it is the safest way more so than certified mail. The thing about Registered mail is there is a chain of custody that is admissible if ever needed in court. Not to say it would be in this instance, but for the general knowledge using registered mail is about as secure a way to move anything you can not hand over in person. I used to delver a business route that had jewelry stores and attorney offices and I had to sign registered out from the secure cage all the time and sometimes having a million dollars worth of gem stones in my pouch was a little unnerving. Especially should I have lost it, I had to pay for it. Tends to make one very very careful.