WHOLESALE PURCHASE ORDER

I own a retail store. I placed a wholesale order with a bedding manufacturer over the phone. They faxed me an invoice. I had to cancel the order because business was unusually slow. When I called to cancel the order the bedding manufacturer told me that I had to pay a 20% restocking fee. That was the first time that they mentioned a restocking fee. At no time prior to my canceling the order did the company mention a restocking fee for canceled orders. They did not mention it orally or in writing. I told them where they could put there fee. They recently sent me an invoice for the 20% restocking fee via certified mail. I don't think that I owe them anything. My question is this: How do I handle this? Do I ignore them? Should I call them? Or should I inform them in writing that their restocking fee is bogus?

I am located in California and the bedding manufacturer is located in Pennsylvania