The safest method is USPS registered mail. It can take much longer to arrive, but it will be under lock and key when it is at a postal center, and anyone who touches it has to sign for it. I know many people who have used registered mail to send diamonds to be reset, for example, and I have, too. They'll give a normal priority mail delivery date, but you have to ignore that. I usually see registered taking 1-2 weeks to arrive. If you want to do it this year, I'd do it before the holidays. I think splitting them into two packages is probably a good idea anyway, though.