I am not sure if this is easy/possible but I'm sure it would not be an uncommon requirement!?

What I want to do is to offer customers a discount depending on their payment method.

To be precise, I want to automatically discount a product by 5% if the customer chooses to pay by bank transfer/cheque rather than paypal.

Does anyone know if there is already a way to do this (mod or plugin), or can anyone suggest a way of doing it.

TIA