Yet their checkout system had no problem taking the code and charging customers’ credit card?

Which is a problem going by this statement in the APCmag article, Dell backtracks on ‘accidental’ 55% PC discount:

Teresa Mullan, from the NSW Office of Fair Trading, told APC that “Whether or not a trader is obliged to complete the transaction will depend on whether a binding contract has been formed. Generally if consumers have not paid for the goods (ie, credit cards debited) one of the elements which constitute a binding contract, that of ‘consideration’, has not been met and the trader may withdraw the offer.”