Adding a surcharge is against your merchant account agreement. A search of the net will yield all the credit card issuers' position on surcharges. For example:

"Why can't I charge a surcharge?
Can the government charge a surcharge?

Surcharging credit card transactions is strictly prohibited by the Card Associations. A merchant may, however, offer discounts for cash transactions provided it is clearly disclosed to cardholders as a cash discount and the cash price is presented as a discount from the standard price available for all other means of payment. MasterCard® modified its regulations to allow the IRS to assess a 2% convenience fee. No other industries were part of that modification. Visa® is not part of the IRS program and has no exceptions to surcharge prohibition."

So...anyone asking for a premium over the listed price is in jeopardy of losing their merchant account. Paypal's policy on surcharges is very clear as well:

"Surcharging for all PayPal payments is prohibited outside of the United Kingdom. According to our User Agreement, sellers residing in the United Kingdom and listing items for sale on a UK-based website may impose a surcharge, but only under the following conditions:
Both the buyer and seller reside in the United Kingdom
The purchase price is paid in Pounds Sterling
The surcharge imposed by the seller is no greater than is necessary to recover the receiving fees incurred by the seller
The seller clearly indicates to the buyer prior to the buyer's submission of a bid or (in non-auction transactions) prior to the completion of the purchase, that a surcharge will be incurred and the seller must indicate the amount of the surcharge"