Storage Shed Company placed me in collections

I had a storage shed. I called the company and cancel, I also sent them a letter. I never went in to pay the last months rent. The next month they charged me a late fee and the following month's rent. I told them I would pay for the last month I was in the shed and the late fee. They would not take a payment unless it was for the FULL amount. I explained I canceled. They said because I did not fill out their cancelation paperwork and pay up to date I'm still a tenant. This continued for 3 months. With Liens and late fees it has totaled $561. I owe them $65 plus $25 late fee. They won't take a payment. Now they have sent a collection company after me. I do not want this to effect my credit.