According to the USPS, it's $3.75 First Class with the zip code you gave me. Weighs 5 ounces. The more cards you buy, the more you'll be able to save on shipping. Best of luck on what you decide. :)
I just grabbed 3 decks to get a quote and from what I can find, First class would come to $5.50, providing I picked the right decks. lol If not, your best bet would be to use those flat rate $7.20 box and I'm sure I can fit more than 3 in there. :)