The extension Discount/fee by Payment type allows to set discount or fee on payment types selected on checkout.
Features:
* allows to set discount or fee based on payment types selected on checkout
* allows to set amount in percent or direct value
* allows to set cap of the discount/fee
* allows to limit for specified range of total only
* allows to set message string in module's admin page with multi-language support
* allows to set categories where discount/fee is accounted

Demo: http://oc.sad.lv/admin/index.php?route=total/discount_by_payment
user/pass: demo/demo

opencart 2.3+ Ocmod version