Look at the agreement that you signed when you rented the unit. They can charge what is in that contract. By signing it you agreed to their terms. If they are charging you more than is provided for in the contract you can address that with them. Its a good idea to evaluate the value of the items you have in storage vs. the cost of storing them. If the cost of storing them exceeds their value to you and in addition you are unable to pay timely perhaps its time to dispose of them or relocate them.
Disclaimer: The foregoing answer does not create an attorney-client relationship with Attorney Cannella or her firm. This answer does not constitute legal advice, is provided for informational and educational purposes only for persons interested in the subject matter, is not legal opinion, nor confidential in nature. Each situation is fact specific and may be subject to state specific laws. Without a comprehensive consultation and review of all the facts and documents at issue it is impossible to evaluate a legal problem fully.