You are having a contractual dispute over your storage unit. The fact that you kept tax records in there which you now need is really of no concern. If I were you I would try to find a way to amicably settle the matter so you can be done with it. I don't know if there is an important reason at the moment that you need the tax records but that is probably way more important than a tiff over a storage unit.
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One option is that you can pay the bill, get all of your items out of storage and then sue the storage facility in small claims for your money back. That will get you your tax returns, end the problem, prevent future charges and a lien sale of all of your items. Pay the amount with a dispute letter explaining you are only paying to stop the process. Then after that is done, send a letter by certified mail, return receipt requested demanding your money back and then go to small claims. There are attorneys who will do the letter(s) for you at affordable prices. Good luck.
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