Various authors wrote:
I assume that there is no problem at all with selling 1kg of sugar
that is also labeled as 2lb 3 1/4oz

Correct. For products controlled by the FPLA, both must be present.


Sugar is usually sold in multiples of 5 pounds so they would have
to buy three tiny bags if they do it your way.

The container size and the text on the label are related but distinct
issues. The FPLA law only relates to labels. As far as I know, sugar
conainer size is not controlled by law.

If a manufacturer/importer offers a product with a metric-only label
and customers will not buy it, that should be up to the marketplace.

You can read what manufacturers are saying if you look for
'Permissible Metric-Only Labeling' on the page:
http://ts.nist.gov/WeightsAndMeasure...c/mpo_home.cfm


A lot of stuff sold in the US already has the metric equivalent marked
after the pounds, ounces or fluid ounces.

Yes. That is because the FPLA controls most (but not all) of the
prepacked things in the supermarket.

Milk is controlled by the USDA. They mandate non-metric labels. Metric
is merely optional. Metric-only milk labels are illegal.

Wine and liquor are controlled by the TTB. They mandate metric labels.
Non-metric only labels would be illegal.