I'm lucky, I already had a digital fish scale used for ..... FISHING! It measures to .1 lb which is accurate enough for the whole bike or even wheelset.

I use the postal scale at work for individual items I buy - saddles, stems,handlebars, etc. More for interest than anything, although I have sent some info into weightweenies. It measures to .1 oz which is about 3 grams - for a 50-300 gram item, that's plenty close.