Delivery services

I ordered quite a few things from Amazon and its sellers, so I gained some experience in how the thing are being delivered. First, I bought a headset and books to quality for Free Shipping. It should have come in one package, but instead, it was delivered in two (both for free). The headset came first and very fast - it was basically next day. Books took more time - about 3 days. Then I bought bags for my bike. This took one week to deliver, because it was sent from the East Coast to the West Coast, while the previous packages were sent from Nevada to California - not a big distance, really. All three shipments were done by UPS. You can track UPS packages online in basically real time - you can see where it is, when it was picked-up etc.
Finally, I ordered watches. There were in stock, so I expected them to be shipped quite fast. The first information was that it will take a week just to ship them (give to the carrier). That was bit of a shock - I was really hopping to get them earlier. Fortunately, they have been shipped in two days.Instead of UPS, the USPS is used and the Amazon shows expected arrival on October 15th! I guess it will arrive earlier.
Overall, it seems that the Amazon is slightly overestimating the time it takes to deliver a package. I guess they use the least optimistic estimates in order not to make the customers disappointed. But I'm also wondering whether the use of carrier depends on the amount you spend and how long you are a customer of Amazon.I prefer the Free Shipping, but first few packages arrive very promptly. Now, it is getting worse. I'm wondering whether this could be intentional - make you used to good service, then deliver the bad service for free, hoping you will prefer to buy the better service next time.
It makes sense, especially because I know that when I finally decide to buy something, I want it as fast as possible! The decision itself can take couple of weeks or months (or a year), but I want the delivery itself to be fast. Well, I don't want it badly enough to pay for, though.