undisclosed charges

[chensium]

I live in Northern California. I recently purchased automotive software from an Australian company. They charged me for the product and nothing else (no shipping). I later received the software along with equipment required to install the software into my car. Then they asked me to mail back the equipment at my own cost. I said that I would not pay for the shipping as it was never listed in my purchase agreement nor was it in my bill that I received and paid for. They said that it was not listed in my documents because they almost never ship out this equipment. They then threatened to charge me ($400) for the equipment.

I've already asked my credit card company and they've confirmed that unauthorized charges will be disputed. But I am wondering if I am legally responsible for the return shipping charge even if it is not mentioned in my bill or purchase agreement.