incorrect package

Mondo company called me to offer me data sim card for MTN, when they delivered was wrong offer, the sent me two sim cards and I canceled it with in hours due to wrong offer. Now Mondo saying they cannot finalized my cancellation requested due to I owe MTN, how on earth I am owing MTN with the offer that I rejected because was wrongly offer. I sent them email now and then to let them to clear that amount on my name. They sent me as follow "Kindly be advised that though you contacted us within cooling off period to cancel MTN has found usage on the sim cards which will have to be paid for by yourself before they can cancel the sim cards." What exactly they want me to pay for?