Not sure what state you are in but in the following states it is not legal for merchants to add surcharges to any credit card transactions: CA, CO, CT, FL, KS, MA, ME, NY, OK, and TX. In addition, most card holders prohibit this as well.
There are exemptions (including some government agencies), but here is what I understand to be the rules for most situations:
A convenience fee cannot be assessed in a face-to-face environment
A convenience fee cannot be assessed for recurring payments. The convenience fee was designed for one-time payments and not for payments in which a cardholder allows his credit card to be periodically charged for recurring goods or services. Examples of recurring charges include, but are not limited to, insurance premiums, subscriptions, Internet service provider monthly fees, membership dues, and utility charges.
The merchant must provide a true convenience in the form of an alternative payment channel outside the merchant's customary face-to-face payment channels, and the fee must be disclosed by the merchant to the cardholder as a charge for the alternative payment channel convenience that is provided.
The convenience fee must be disclosed prior to the completion of the transaction, and the cardholder must be given the option to cancel the transaction if not wanting to pay the fee.
The convenience fee must be included in the total amount of the transaction; it cannot be split out from the transaction amount.
If a convenience fee is assessed it must be for all payments (V, MC, Discover, AMEX, ACH, Check) within a particular payment channel (mail, phone, internet).
Since all convenience fees are required to be assessed equally, a merchant who accepts Visa is restricted to a flat convenience fee for all cards.
Theses rules may or may not apply to your situation, and I am no expert on these regulations. This is just info I have have gathered from family members who work with credit card companies and from my own research. If you really want to pursue it your best bet would be to contact your card holder.