I am thankful that you have accepted my idea (or otherwise someone else’s idea) of re-purchasable card backs in the shop, however i was expecting it to rotate much more frequently, which i would suggest a rotation every 10 days as quests along can provide enough coins for the purchase.
Moreover, if you would like to keep the current pace, may be give me 3 card backs to choose from instead of just one.
It would be somewhat at the expense of the rarity of card backs. For some users, a card back from the early days of the game can be very meaningful. It’s an aspect with which they can distinguish themselves from the relatively new users. Speeding up the routine you specify ensures that fewer starting points are left for users to remain unique in the game.