I ordered an ABS (plastic) threaded plug for a clean-out pipe next to my pool. I measured the one I mangled to remove, and it was 3.5".
The one I received must have been for a 3.5" inside-pipe-diameter, because the plug measures 4" across.
After wrestling with whether the $3.70 (incl. HI Tax) is worth the hassle of returning, I decided to request the refund -- mainly to see what the Amazon refund process would do.
Surprisingly, instead of making me print a label, pack it up, and take it to the local UPS drop-off, the site said:

I'm sure they have some sort of controls to prevent abuse, but imagine ordering a bunch of < $5 items and having the cost of each refunded after delivery -- just because you can keep it for free!
I'm sure your can cancel the refund if the site continues on with the return process, but just checking to see might be worth the trouble.
Stealing is bad. Gifts are nice. Information is power.
