Hello Yukiko,
 
Yes, if you have earnings in your account you can use those to pay for cards.  After you select the cards you wish to purchase and click on the shopping cart at the top of the page, you will be taken to a 'view cart' page where you can enter your artist promo code and just below that you will see a clickable link in blue which says, "use my artists commission to pay for this order."  and will show your balance in your account.  Click on that and it will take you to the checkout page where you can finalize and pay for your order.
 
Hope that helps!  I took a look at your cards and store, you have just a few cards, but they are all exceptional.  Beautiful designs!
 
Best always, Randy

Thank you Randy for your help and kind words! And Sue, thank too!