no point disputing the credit card charge because it was agreed and signed for!

this was not some retrospective charge applied by the airline. They gave you a price, you agreed to it.

the price charged was 1.5% of the [full IATA published] economy fare, which would be many thousands of dollars.

You cannot argue a 'mistake' because the charts they show are for europe and the USA. Assume all other destinations the 1.5% applies.

you cannot argue 'per bag' because they calculated it and gave you the choice and you accepted it.

to assume airlines would give you 2 bags free just because american carriers do is quite an assumption!