My Tech Discount card says there is a 3% reduction for things needed, but when I play it, the items (tin cans, barbed wire, etc.) are not reduced. The alloy requirements are lowered. Should the items be reduced, too? Perhaps the information on the card needs correcting.

You only get a reduction if the percentage noted on the artifact is equivalent to at least one alloy/item. For 3% if a tech requires less than 33.33 alloys or items, then you won't see an actual difference. if you want to see a reduction of 3, the base cost has to be at least 100.

Note that other factors that influence tech costs (like the tech discount advancement) can make this even more complicated to calculate. But I use this artifact and it worked correctly all the time for me, so you are probably just dealing with "cheap" techs (meaning, that you only need 20 tin cans or so, not cheap in regards to the sell value of the required alloys/items).

Edit: This is applied to each tech cost separately. If a tech costs 100 nickel bars and 3 tin cans, you will just see a reduction of 3 nickel bars but none for the tin cans, because there are more than 33.33 nickel bars but fewer tin cans.

Edited 9/13/2021 18:13:50

That was probably the problem. It is working correctly on an update requiring 199 tin cans.