i also order a lot from amazon, and i used to get my orders within 3 days, whether or not i ordered it direct from amazon or "used" from a third-party seller. for a while now, maybe as long or more than a year, i have noticed that the items are taking longer to get here. but that is also reflected in their "estimated delivery date" that you can see on your account page, recent orders on amazon.com. even though it takes longer, i am still happy.
the reason i shop on amazon is because i can things so much cheaper than most places BECAUSE i always order used, and almost always the cheapest that they have, because we are broke!
IF you buy let's say 5 items. 2 are from AMAZON, 3 are from a third party seller. your order will be broken down into 2 different batch shipments. let's say you order 5 items ALL from different third-party sellers, then your order will ship from 5 different people in 5 different places.
BECAUSE anyone can sell on amazon. if you have lots of books, you can sell. when some buys from you, you will obviously ship their item from YOUR TOWN.
so, depending on what you ordered from whom, this COULD be an explanation. but i don't know specifically what y'all ordered.
ETA: and ALL my orders in my buying history have been delivered USPS, BUT i have a package from amazon that will be here any day, so we'll see how that one comes in!