Amazon has created a level of customer expectation that seems hard for them to meet. They charge Prime customers a fee and then promise 2 day delivery but seem almost always to fall short. At least here in the Greater NYC area. I have spent some time trying to unravel their process using the USPS and think I have some detail.
The following is my best guess based upon a few hundreds of samples.
The above is a flow example. This is a case where they get a product from a third party vendor, they do not have it on hand. Often there is a delay here. The vendor connection and transport is subject to poor execution and thus begins the collapse of the two day promise.
Once Amazon "takes" possession they assign a shipping number from USPS. But apparently it is still a long time before USPS takes custody of the product. Amazon moves it to a local shipping point, one of theirs, then assigns it to some other third party transport to take it to a local Amazon site near the delivery. For us it is in Avenel in NJ.
At that point they contact the USPS for local transport. Up till that point one tracks Amazon NOT USPS apparently. I believe that USPS picks up and distributes the package to the local USPS where sometimes tracking data may be recorded. After all this is NOT UPS, it is the Government so don't expect people to follow through as expected.
Then the USPS delivers the package, hopefully, to the right address and not left in the rain, snow, or the wrong address.
This may be a low cost solution but it is highly prone to faults and failures. No longer is 2 day 2 days nor especially is next day ever next day, despite the extra fee charged.
My personal opinion is that thus whole system must be redesigned. It is a mess and will soon cost Amazon customers. In addition one may initially assume delays are due to the USPS but it appears that they do not take custody until delivered locally and that the tracking is just to keep the customer happy, albeit delayed.
There may have to be some shakeup at Amazon. Customers will not tolerate extra payments for poor service. At least I believe so.