Might be worth doing a search on ebay for a good used commercial scales. These are calibrated and checked for accuracy. Here in the UK the name to look out for is Salter, but the principle brand may be different where you are. A used set of good commercial scales will no doubt outlast and be more accurate than many new scales made for fishing or other sales to the public - perhaps even archery scales. Sadly with so many small stores going to the wall there are plenty around.