I haven't bought a decent postal scale yet. I use a food scale, to 20oz, but only consider it approximate. I use volumes of water as a reference weight for calibrating.

For heavier items, my digital bathroom scales really suck, so I sometimes rig up a balance scale, using a long stick, and water weights on one end and clothes or gear on the other. Make sure when you do this that you don't pull your dining room chandelier from the ceiling.