Hello,
This should be as simple as entering the artist promotion code into the Promo Code box on the In Cart page.  It will display a line item discount in your total amount due in red and you should see this under the promo code box confirming it's been applied:
Special $1.70 pricing for artists  
 
In addition, you can apply your earnings to pay for / a portion of your balance due via this just below the promo code box:
» Use my artist commissions to pay for this order   ($XX.XX available)
 
You should see this under the promo code box confirmation it's been applied:
Apply earned commissions ($X.XX)
 
As orders have shipping, handling, postage fees there may still be a nominal amount due if your commissions amount does not cover it.
 
We will take a peek at some of your orders and email you as to what we find.  
 
please note: this is posted from my artist forum id vs my GCU Community Manager admin id due to technical access issues

Ok, posting for all as the process is working as it should.
 
The order summary does look odd that it does not show the reduced price for the artist promo code and only shows the commissions amount removed.
 
For example:
 
Order Total for XXXXXX
Subtotal: $3.89
Tax: $0.15
Postage: $0.63
Discount: $4.67
Total: $0.00
 
The $4.67 line item discount is a COMBINED amount of the card price discount AND the applied commissions. That would be $2.19 discount for artist card price ($3.89 - $1.70 = $2.19) plus $2.48 applied commissions (1.70 card price + .15 tax + .63 postage).  
 
You can see the breakout by going to your Order History and looking at the details of the order.  Here for example:
       Used Coupons      Discount        
     Special $1.70 pricing for artists (promo code) $2.19        
     Apply earned commissions ($2.47) (promo code: commission)
 
Let me now if you have additional questions.
 
please note: this is posted from my artist forum id vs my GCU Community Manager admin id due to technical access issues