Low interest rates have shrunkcredit card profit margins, pushingsome issuers to beef up other revenuestreams, as some unhappy cardholdershave found out. One of the mostcommon methods credit card companiesuse to put more muscle into theirbottom line is to jack up fees that cardholders pay when they are late with apayment or go over their creditlimit. Late charges, which now accountfor almost 40% of credit cardcompany income, are now as high as$35 and industry watchers predictthey'll go higher. The total take fromlate charges will hit $13 billion thisyear, according to some estimates. Atthe same time, customers are gettingless time to pay their bills; the averagegrace period is now 20.6 days,down from 27.8 days 10 years ago.