I learned a lesson the hard way with USPS. I sent a cashier's check from No Cal, to So Cal. I sent it by certified mail. After 2 weeks, the company still had not received the check. I went to the USPS, and was advised that they could do a search, but they wouldn't do so for 30 days after mailing.
I stopped payment of the check with my bank, which cost $15, and they advised me that my account wouldn't be credited with the amount of the check for 30 days after the stop payment.
I obtained a replacement check, and sent it by Registered mail. This has an unbroken chain of possessors, just like evidence. It costs $6, but is worth it.
With split second bureaucratic efficiency, BOTH checks were delivered the same day, about 3 weeks after the first mailing.
Use Registered mail, even for objects with no intrinsic value. Certified mail only helps you IF the object arrives, not if it gets lost.
I have used UPS and Fed Ex as well, but the PO is a lot closer and easier to use for me.
Hope this helps, Walt