Century Manufacturing Corp. Offering "free samples" then sending a bill Seaford New York

Last year, my company (I do accounts payable) received a phone call about an unpaid invoice from March 2011  (prior to when I was employed). I looked for the invoice in the old vouchers, and could not find it. I called my sewer superintendant, who supposedly ordered the product (degreaser) and he had no clue what it was. I went ahead and paid it. Well, this March, my sewer superintendant received a "free" sample of degreaser from Century. The invoce I received here was for 2 buckets, when they only received one. I then got another invoice the next month for another 2 buckets of degreaser. I paid both of those invoices. Not even a week later my sewer superintendant called and told me "Don't pay anything from Century Manufacturing. They sent me 1 bucket as a free sample, and now they keep sending me stuff and won't take them back. The stuff doesn't work anyway! I just refused delivery on another 2 buckets today." Since I had already paid them for the previous 2 deliveries, there was nothing I could do and chalked it up to our company being out $900. Fast forward to last week... I get a phone call from Century stating that I had an outstanding invoice. I told them I would call them back after I discussed the matter with my sewer superintendant. I called him and he said they refused shipment on the last delivery and had two buckets still sitting there they could have back. I called Century back and got a voicemail message. The representative never called me back. I got another call today and I told the rep that we had paid for 2 shipments, which we never ordered in the first place and refused delivery on the shipment in question. I also informed him that we had 2 buckets that we tried to return. The rep got angry with me and told me that we had ordered these products and had we tried to return them in April when we received them (which we tried to do), they would have taken them, but now it is too late. I told him I would not be paying the invoice, considering this is the first I had heard about the invoice in 6 months. I assume we will be sent to collections, but I am not paying another $600 to this company for stuff that doesn't work and stuff we never received.