I bought a product but got lost when I shipped back to the compa

Hi,

A few month back, I bought a product from a company. Then I returned the product within the try out period. After quite a while, the company replied back that they never received my return and asked me to pay the full amount. The worst thing for me is that I forgot to retain my shipping receipt long enough and I could not find it any more. Now a debt collector begin to send me letter for this issue.

Please let me know what my liability should be at this point?

Thanks in advance!