American Express is acredit-card type used under the existingCREDIT_CARDpayment method, rather than a new payment processor or Site Preference. Therefore, the appropriate Business Manager action is to manage the supported Credit/Debit Cards associated with the credit-card payment method.
Salesforce documents that merchants manage payment methods underMerchant Tools > Site Ordering > Payment Methodsand that the Credit/Debit Cards page manages card types specifically in the context of the defaultCREDIT_CARDpayment method.
A payment processor represents the integration that processes transactions; American Express itself is not created as a payment processor simply because that card network should be accepted. Likewise, Site Preferences are not the mechanism used to enable supported card brands.
The underlying payment processor must, of course, support American Express transactions, but storefront card eligibility is configured through the credit/debit card configuration associated with the credit-card payment method. Card configuration can additionally restrict cards according to countries, currencies, customer groups, or payment ranges.
Study Guide reference:Data Management Using Business Manager Usage — Payment Methods, CREDIT_CARD, Credit/Debit Cards, card types, and payment processors.
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