Merchants generally will need to pay ~3% to accept a credit card transaction, however this is not their only cost associated with accepting a credit card. Anytime that a merchant ships/delivers a product and later faces a chargeback claim, will have costs fighting the chargeback regardless if it is successful or not. If the transaction is actually chargedback then they will have provided a good/service and received nothing in return.

Also who is to say that a merchant wouldn't offer a similar discount to what it costs them in CC fees? Even a 3% discount would likely be sought after by consumers as few people are willing to pass up a deal when they otherwise would be purchasing something