Heyo.
On the back of the World of Warships giftcards it does indicate that they contain a value of doubloons and that they are redeemed for this amount. The card does not indicate that it can be used on the premium store for further purchases.
While I can completely understand where you're coming from, currency cards (like visa giftcards/karmacoin cards etc) require a considerably larger database system to manage. The system we use with the doubloon cards is not dissimilar to how other games and companies handle the cards. The good news is, we do offer majority of ships/items available in the armory for doubloons and intend to continue expanding this offering going forward.
In future, it should be noted, that Karma Coin cards are a great means to do transactions of which you've described. I personally use Karma coins when making gift transactions for the other regions.
Fem,