As far as guaranteeing delivery...Tracking is important to me.
I like using the USPS for shipments I send and like it when anyone sends to me. I think their tracking system is much more thorough so you understand where the package is most of the time. However I was burned on this a few times recently. When sending a repair out to a store that was not open on Mondays, I marked the box saying "The store is closed on Mondays", they still tried to deliver, marked it in their system as delivered and then delivered it the following day!
On my end, one train was marked in their system as "delivered/at the door" yet, I looked out and there was not any delivery truck on my dead end street. An hour later it was actually delivered. Had a similar experience more recently with T-shirts. They were marked as delivered on a Tuesday when it was fairly icy out. The local post office for 2 days did not pick up the phone (I did not feel like taking off from work to visit them)...then 2 days later on Thursday it was actually delivered.
I would call these exceptions and still say, if the scanning is done correctly, I think the USPS is far superior as far as tracking goes. You can sign up for email and/or text alerts if you have the tracking number. Ideally, the tracking would tell you each instance of when the package arrived at and left each mail facility...again, but only if the workers stick to doing the scanning correctly.
You can also request a signature, but that sometimes annoys people.
Tom