Write down -what- result.

You put the bag on the scale, it then sets this amount to 0.0

You put the product on the scale, (say 500g of apples), It shows 500g.

You remove the bag, it takes off 4g, you add the bag it puts on 4g.

There is no need to write down the result.

In the US, produce is rarely weighted and labeled in the produce section with the bag. Only at checkout is it weighed for sale, so no opportunity to tare the container.