Can I charge a credit card fee on US online orders?

Only on credit cards, never on Visa debit or prepaid. Under Visa's rules the surcharge may not exceed your discount rate for that card or 3%, whichever is lower, and you must notify your acquirer 30 days before.

• Credit cards only: Visa debit and prepaid cards cannot be surcharged.
• The cap: your discount rate for that card, or 3%, whichever is lower.
• Tell your acquirer at least 30 days before you start.
• Disclose it: at the point of entry, at checkout and on every receipt, online too. State laws still apply.