In March, a customer spent $18.00 at an office-supply store for some 12-packs of pens. At that time, the office-supply store was selling packages of one dozen pens for $1.95. Any customer who bought more than 4 packages, received a discount of $0.25 on the price of each package in excess of 4 packages. In September, the same customer went back to the store to buy the same number of pens. By then, the price per 12-pack of pens had been raised to $2.10 each, with the same discount for more than 4 packages. How much more did the customer have to pay in September than in March?​