require payment method even for free items

Hi,

There are checks a bit everywhere for not displaying payment methods when the order amount is 0 as most payment gateways do not accept transactions for 0.
So the modifications won't be easy. You can search for conditions on prices[0]->price_value_with_tax but that should yield a lot of places, related or not to that...

What you might want to consider:
not have payment plugins
use the custom fields of the table "order" (business edition) to ask for the credit card information on the checkout. The information will then be stored in the order on the back end.
The cons : it's not a secure way of storing credit card information.
The pros: it's really easy to do as there is no code modification.