It's taken me well over a year to figure it out, but:
If you are shopping for some widget, find it on Amazon.com and put it into your "cart".
But don't buy it yet.
Wait a few hours. Or days. Go back and look again. Scroll down.
You'll find a whole mess of
the same item listed for less money.
Every time.
So you pick out the cheapest of all the options shown, and put one of
those in your cart.
Wait a few hours. Or a day.
Go back tomorrow and there are options for even cheaper!
Above is a textbook example.
I'll check it again tomorrow.
If it drops below $30.00 I'll buy one.
Otherwise, I have to drive across town to use the one I gave to my buddy a few weeks ago. Only need it for one thread on one widget.