I suppose the Postal Service really does deliver most of the mail successfully, but if they didn't deliver something to you, how would you know? Most of what we get is junk anyway, and would never be missed.
In two recent cases, though, I did miss things. Since we're on the move all the time, we have a mail forwarding service. Every few weeks, we call them up and they send us a priority mail package containing all the US mail they've received since the last time. We were in a camp in Moab, Utah, for a week and ordered our mail to be forwarded. It went out on Tuesday, via priority mail, which is advertised as 2-3 business days delivery. It even has a tracking number, and when we looked it up, it said expected delivery was Thursday.
When Thursday came and went, we looked again and expected delivery was still Thursday, and the package had made its way from Livingston, TX to Houston, to Phoenix, and to Salt Lake City, where the latest scan indicated that it was received at the sort facility on Thursday (no time stamp).
Moab is not too far from Salt Lake City, so we figured maybe it would arrive Friday. No luck. No indication of any more scans on the tracking web site. The Moab Post Office confirmed that it had not been received on Friday after the local delivery trucks left, and they get mail from the sort center in Provo, which would be the next sort after Salt Lake City.
The post office web site does have a way to send inquiries about such things, and it says to expect a response in 48 hours. Oh, and if your priority mail hasn't arrived in 2-3 days, wait until 5 business days and let us know, and we'll start looking for it then.
We made arrangements with the Moab office that they would call us Saturday morning before we left town, and let us know if our package had arrived. If not, we could submit a change of address form and it would be forwarded, and if so we could pick it up before it went onto the truck. It did arrive Saturday morning and I picked it up at the post office before we left.
Another thing we were watching for were some rebate checks, which were due because of getting our new smart phones and personal internet device. Each one came with a separate rebate. On April 10, the rebate company mailed three checks, all to our Livingston address. On our next mail shipment, one of the checks arrived. We kept asking our mail forwarder about the others until the 60-day waiting period expired at the rebate company, and then ordered replacements. They told us one of the checks had been returned by the Post Office marked "Attempted - Not Known", and the other one had not cleared the bank. Three identical items mailed on the same day to the same place, one delivered, one considered undeliverable, and the third simply disappeared (into Newman's closet?). Had they been some pieces of junk mail that we weren't expecting and didn't care about, we might have thought the Post Office had delivered everything that had been mailed to us.