Darlene's Diner accepts American Express® credit cards from its customers. Darlene's is closed on Sundays and on that day records the weekly sales and remits the credit card receipts to American Express. For the week ending on Sunday, June 12, cash sales totaled $2,430 and credit card sales amounted to $3,500. On June 15, Darlene's received $3,360 from American Express as payment for the credit card receipts. Prepare the necessary journal entries on June 12 and June 15. As a percentage, what collection fee is American Express charging Darlene's?