Hi AK2
The fees charged by MYOB in relation to Online Payments can be found on Help Article: Fees and charges.
Currently, these chargers are paid to the MYOB Customer, not the business customer i.e. sender of the invoice is charged, not the recipient. However, as mentioned on the popular idea on this topic: Online Payments: Credit Card Surcharge, we're committed to building out that feature - no date has been set for that at this stage.
In terms of overseas credit cards, in theory, any VISA or MasterCard or AMEX credit card can be used by the system. The system will use a 3DS-secure system. As such some foreign credit cards may not meet that criteria and thus not be accepted by the MYOB Online Payments system.