Can I charge a customer a merchant service credit card fee on top of their invoice?

Our customers rarely use credit cards as a form of payment. It's usually a check via mail. But a few do use a Visa or MasterCard once in a blue moon. We are charged 2.75% from our bank for the the total of the credit card purchase. At the time of the credit card transaction can I add the standard percentage on top of the invoice amount? Am I legally allowed to do so? Also, am I legally allowed to markup the 2.75% we are charged on our merchant statement?