Print-on-demand (POD) products work a little differently than products printed in bulk and sold from inventory. Here’s a comparison to illustrate:
For a traditional print run, I would calculate the final price of the deck (including per-deck printing costs) of $20. Then, I would set the “claim your copy” reward tier at $18, effectively creating a preorder with a small discount.
For POD, I can’t collect that print cost up-front like that; it has to be paid when you tell the vendor to print your individual copy. So instead of asking you to pledge $18 all at once, you only pledge $8 for the Kickstarter, and the other $10(ish) is paid directly to the vendor when you claim your reward.
As you can see, the total cost for you to get your deck is the same in both cases (and it’s less than someone buying it at retail later!), it’s just that the quirks of POD mean the process is split into two payments instead of a single lump sum.