There's mention in this thread that the US Post Office Priority Mail provides a "2-3 day delivery guarantee". There is not such guarantee. The 2-3 days is strictly a target time for delivery.
In my experience more times than not delivery of Priority Mail takes longer and sometimes considerably longer than the spec'ed 2 or 3 day timeframe. Whenever I have brought this up with the Post Office their response is to use Express Mail if you want a guaranteed delivery time. More or less, Priority Mail is a rip off.
And talking about shippers. I used DHL to ship some old stereo cartridges. Paid extra to insure the contents for about 1/6 of their cost if new. DHL lost the box and refused to pay my claim. After about months of round around and lies from everyone within DHL, they finally agreed to pay. But not for the insured value.
I provided documentation of the retail cost. They said my 1/6 of retail insured value was meaningless. They would but pay my wholesale cost regardless that the insurance contract stated no such thing. After explaining I am a consumer with no access to wholesale, DHL insisted that some business at some time purchased them at a wholesale price, that I had to go back to them and get documentation of that price. How many businesses will provide consumers with documentation of their wholesale costs, especially those that have been out of business for years as was the case with me? Going to the top person at DHL did not help. My resolution was small claims court.