AnthemRO survives on donations. Therefore, to thank players for their generosity, we match their donations with 'credit' that they can use to get items via our Item Shop. So if you donate $1, you'd have $1 Ad Credit. You can vote for us (and provided you're logged in) and earn ad credit without donating actual money.
On AnthemRO, players can trade/sell/buy ad credit ingame. You can withdraw your $1 Ad Credit into an in-game item called pennies. So 100 Ad pennies = 1 Ad Credit ( =$1). The price of the pennies is determined by the player economy. So if $1 Ad Credit (100 pennies) costs 800k zeny, then you can sell loot and use zeny to gain Ad Credit, without donating actual money! You can transfer your ingame pennies back into Ad Credit that you can use in the Item Shop, or for services such rollbacks, renames, etc.
Therefore you can save up $10 Ad Credits, to purchase a Goblin Archer Card. If an item isn't listed with a specific price otherwise, and isn't in the Unavailable Items List, then its default price is $10 Ad Credits.
Phew.
Or you can try your luck and camp goblins some more. Good luck!