Since I cannot tell what transpired between you two since you hid the comments of them asking about the status of their commission, I can only gather that they only asked where it was - not that they demanded you do it right then and there. So rushing it to give Marble what they wanted is a slipshod excuse because a proper update and a follow through to pay the same attention to their piece as you to your others would most likely have sufficed for the moment.

Same with the refund, it is not uncommon for an artist to refund a customer when they choose to no longer work with them. They refund and more often than not sell off the piece after reworking it to something newer than originally done. So those hours of work would never be wasted if you recycled it properly.

And honestly, its not just the artwork itself, but the experience you created WITH it.