I bought a pump from this sham of a company about ...

I bought a pump from this sham of a company about 1.5 years ago. They told me it would be free due to insurance. After delivering it the price changed to $3000. I have contacted them many times and they have said they made a mistake, but they keep on sending me bills and making me call them again, with the same result. Now they send me a bill for $6000 saying they sold me two pumps (they sold me one). They are threatening a collection company if I don't respond within 7 days, even though this is from 1.5 years ago. Stay away from this company!!!