Hi.
I would have to understand a bit more about how your items were set up to be of any more help than offering a link to the documentation for Coupons.
From reading the documentation, coupon settings should allow a set amount ($10) to be deducted from the entire order--with the appropriate code entered at checkout--and limitations can be set to 1 discount per customer/order.
In the first screenshot, I've added the same drawing class to my cart 3 times. When I added the coupon code SAVE10 during checkout, only $10 was deducted from the entire purchase.
In the next screenshot, I added two different art classes to my cart and used the SAVE10 coupon code. Only $10 was deducted from the entire purchase.
My coupon settings:
