First you should have a local attorney look at the contract for you. This way they can tell you if the contract terms are, in fact, what you expected. The attorney would be able to tell you if the decorator is entitled to those charges or not.
Next, if she is not entitled to the charges, there is actually no breach of contract unless she quits or refuses to complete the contract. Simply asking for extra payments isn't a breach, by itself. Something you can do right away is to demand "reasonable assurances." You demand reasonable assurances by sending a letter stating that you do not believe that you are required to pay the extra charges, and ask if she still plans to fulfill the promise of working the wedding without those actual payments. With no response or a response of no to this letter, it would put you on a much better legal ground to receive your deposit back. At this point you could sue for a breach of contract and seek the return of your deposit. You should know, however, that the question of "who breached first" is critical here, and it is usually very dependent on the circumstances of your case. Therefore an attorney should help you prepare the letter, as well as evaluate the contract.
Answered on Sep 15th, 2011 at 7:05 PM