I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
You have the ability to sue the contractor for a refund of the extra $550, based on the agreed contract price and the documentation you have. Your testimony showing what you agreed to is proof, especially since you can show that the over-payment matches the price of one item. He agreed to do the job for a certain price, and your mistake in paying too much doesn't invalidate that agreement.
Because of the amount involved, you can bring the suit in Small Claims Court for both the $550 and your filing fees. Small claims cases are filed in the county where the business operates. This site has links to forms for different counties.
The other option is to send a letter, via certified mail, to the business's address if you have it (or email), giving a firm deadline for returning your money or you will go to court and get an order that he pay the extra money plus your filing fees. Since he has no legal basis at all for keeping the money, it's possible that he'll give the money back to avoid a judgment for more than the original $550. It's also possible that he's stalling because he thinks you'll drop it, so a letter can let him know that you're serious about this.
If you have any questions or concerns about my response, please reply WITHOUT RATING. It's important that you are 100% satisfied with my courtesy and professionalism. Otherwise, please rate my service positively so I am paid for the time I spend answering questions. If you are on a mobile device, you may need to scroll to the right. There is no charge for follow-up questions. Thank you.