Hi Daniel,
The discount rate and transaction fee for amex/diners is collected by the Merchant account. If you are asking to apply an additional surcharge, you can use the Surcharge feature located in the Cart Settings. The surcharge field allows you to enter a percent to be added to the order subtotal. The following screens display the surcharge field:
- Cart screen
- Confirmation page
- Confirm status page
- Email confirmations
- Manage orders screen
Thanks,