I think that's just the way it is. A deck of cards can't be bent and is thick, so it has to be shipped as a package. You will find that the post office has better rates than UPS, FedEx, etc.

You can save by shipping through PayPal, Ebay, or Stamps.com because their prepaid postage is sold at essentially wholesale rates. You will pay full retail if you go to the post office and buy at the counter or a self-service machine there.

CAVEAT: I have not explored whether the "large envelope" (flats) rate might apply to a deck of cards - others may have suggestions about that product. In general, though, it doesn't apply when the envelope can't be bent or when there is more than a 1/4 inch variation