For vast majority of things amazon sells, a system checking if the weight is within a pound or 25%, whichever is greater, would almost never have a false positive.
I checked my order history and my last orders are around:
- 350g
- 600g
- 200g
- 500g
- 5kg
- 120g
a box with empty packing would fit within a pound for most of these. I don't intend to nitpick your back of the enveloppe calculation, just that it's not as simple as it seems.
I don't know if I am the typical amazon shopper, but on my 35 orders in the last 6 months the above pattern is repeating with mainly very small items (like cables, dongles etc.) and one big heavy package from time to time.
At that level of detail, it also requires the seller to give an exact weight for the goods they are selling, and update it each time it differs by even dozens of grams.
It can be done, just not sure the parties involved are willing to commit to that level of accuracy.
