Re: Today some kinds of packages will state the "tare weight" meaning the weight of the container.

yup, deli counters in grocery stores (at least in NYC!) do it all the time.. they have sealed 'tare' contaniners, that they use to zero out the scale, before weighing out your potato salad or what ever!

butchers do it to, they have wrapped stryroform trays, with plastic wrap, and a lable, too! and they have had these for years! (i remember them from when i visited my father in the 1950's &60's, he was an A&P butcher.)