...when I ordered them, I change the decks from range 6-12 decks and the shipping price didn't change.
And this might be the reason of the minimum 4 decks as well, if you buy 4 decks that means you have around 8$ for shipping. But, for 3 or less, the shipping that included in the deck price isn't enough.
There was a problem with their site when they launched this deck. The 4 deck minimum and low shipping was a bug. A brick now costs $15 again, and 4 decks will still cost you $9.
Per deck including the least-expensive domestic shipping:
Fulton's Chinese New Year:
1 deck: $14.88
3 decks (5% off!): $11.10
6 decks (10% off!): $9.83
12 decks (15% off!): $8.79
144 decks (still 15%): $7.97
Okay, $8/deck shipped to your door seems very reasonable. That's even a little less than Kickstarter prices! Well, for one deck, anyway. But then you have 143 other decks to deal with. And if you're going to compare best-case prices… then let's compare to a fully custom deck:
144 Vortex decks, shipped, during kickstarter: $6.94
144 Vortex decks, shipped, during final Vortex sale (and due to the reseller package discount, you also end up with two uncut sheets!): $8.11
144 Revision One/Brimstone decks, from circlecitycards.com today: $5.10
1 Revision One or Brimstone deck, from Circle City today: $11.
Circle City charges you $3 for shipping one deck. Alex charges you $6.75, which is $0.75 more than D&D, but once you hit two decks Alex starts charging you less. Ellusionist for a single deck offers you $6.70 for priority or $3.27 for "SuperSaver USPS". And in these three cases, only the Vortex deck costs more to start with, coming in at twelve cents more for a one-man operation with fully custom faces. And I highly doubt Circle City or Alex have streamlined their shipping methods and processes in the same way that D&D, Ellusionist, and T11 surely must have.
Feels like beating a dead horse at this point, so I'll stop. My point is that, unless you're a wholesale customer, this is a mighty expensive deck for three custom cards, a nice back, and a nice box. It is a VERY nice box, and a very nice back, and what custom faces were done are great!
Buy what you want, and what you like, but always be realistic about the costs involved.