Offshore payment processors tend to use offshore banks for their merchant-accounts. Here the frightening "whitelisting" comes into play. Whitelisting came up in 2014 and allows only certain banks (mostly domestic only) to charge credit cards. Which Banks are allowed to charge a creditcard of a customer is controlled by the bank of the customer. So, as someone with an offshore merchant account, you will have some troubles to charge your customers (not all but some).