Well it goes something like this.
If it fits in the mailbox, that's where it goes. If it doesn't, they'll sit it on the porch or in between the door and screen door (if there's room).
They'll also knock, so you will know they're there.
I've only had one instance where my postal lady sat the box, it had a drawing tablet inside, on the grass when it was RAINING. Made me so mad! But luckily it hadn't rained too hard...
idk about in US but here if a package is over $50 you have to sign for it so they'll deliver it to your door, and if youre not home you gotta pick it up from post office.
everything that i've bought off amazon has been too big for my letter box, but every time i've bought a game online (not specifially amazon) it's been put in my letterbox