American Express has historically charged merchants more than Visa and Mastercard — sometimes 0.50% more on the same transaction. The reason: Amex doesn't use traditional interchange; it sets its own merchant fees directly. For decades, Amex required separate merchant accounts and separate billing.
In 2014, Amex launched OptBlue: a program that lets processors handle Amex transactions through the same merchant account as Visa/MC, with Amex fees that are typically only 10–30 basis points above the comparable Visa rate. Most modern processors (Stripe, Square, Helcim, Stax, Dharma) support OptBlue automatically.
If you have a separate 'Amex Direct' relationship, check whether switching to OptBlue through your main processor saves money. For most SMBs it does.