Since it weighs over 13 ounces, you have to send it priority.
Generally, the Flat-Rate boxes are the cheapest method. The Small box (Outside: 8 11/16" x 5 7/16" x 1 3/4", Inside: 8 5/8" x 5 3/8" x 1 5/8") is $5.15, but you would have to break up the cards into plastic boxes or some other very supportive method to get them to fit. The medium box (Outside: 11 1/4" x 8 3/4" x 6", Inside: 11" x 8 1/2" x 5 1/2") is $11.30.
Even though some people still do it, it is not legal to send cards in Media Mail.
Did the Postal employee tell you what services you would have gotten for the 9 (Delivery Confirmation, insurance, etc.)?