I use a "Royal digital postage scale" The (RC40) model.
It is very accurate, I have checked it with calibrated scale weights and the post office scales.
It is good up to 40 lbs.
You can find them on ebay or Office Max, Staples etc.
I got it because of someone here on RCU who was using it and liked it.

Another option is a refrigerant charging scale, a lot more expensive but very handy and can be found used.