Conservice utility management and billing — extra fee for using credit card to pay bill

This company charges a $3 fee for paying online using a credit card. I had heard that this was illegal, so I contacted my credit card company (MasterCard), which told me it was illegal 99.9% of the time, leaving the .1% to a payment arrangement through a third party agent. In this age of technology and automated payment software, I would think this is not only unethical, but unfair. As for the legality, I'm not an expert in this area, but common sense tells me it should be.

Sincerely, JD