Yes, as long as the card bears the logo of one of the card brands you have enabled. All ClientPay accounts are able to accept Visa, MasterCard and Discover payments but American Express is optional so check your merchant services agreement or contact ClientPay Support if you are unsure.
When it comes to entering the payment, the billing address fields within the portal and on customer facing pages are formatted for US-based addresses but only the street address and zip code fields are required. Zip code is not formatted so you can use 5 or 9-digit numerical codes as well as alphanumeric postal codes in that field. We recognize this can be an obstacle for your clients. Our products team is currently looking at ways to enhance the forms to accommodate international addresses.