MotherNature.com Made Unauthorized Charge on my Credit Card, Internet

Made an online transaction with them (mothernature.com) for an agreed upon amount.  I reviewed the charges and then submitted my credit card info. I received an email confirmation for the amount agreed upon that contained an itemized list and a purchase confirmation.  They then charged my credit card for an amount much more than what was on the receipt.  I contacted the company via email who very unapologetically said that because I was at a PO Box, that merited a much higher shipping fee and that was their policy. 

I don't argue that. I argue that when a transaction is made for a certain amount, a company can't ethically just tack on extra money at will.  The customer service person refused to modify the bill or cancel my order.  They are now ignoring all further email from me.  I have to dispute this through my credit card company which is a hassle.  I have the email receipt they sent with the amount I agreed to pay. I hope this is sufficient enough for proof.  I've never had a company do this to me before.