That's a tough situation but I would think that if the shop diagnosed the creak, sold you a fork, and were the ones to perform the replacement, they should be taking care of you right now. No questions asked. Surely they don't want to eat the cost of the fork which you can bet is a bout half what you actually paid but if the owner is any kind of good businessman, he will do what is right and give you a refund for at least the product.
You seem to be the type to buy nice bikes and equipment so he needs to look at the big picture of having you return to his shop or not. Good luck and be persistent, give the shop owner a chance to do the right thing.
Also, be sure you're dealing with the owner and not a salesman or mechanic as they will only try to cover their ass at this point.