Invoice and tender

Hi guys!
I'm trying to find out e couple of payment options for my shop.
I need to have an payment option for the user to be able to get an invoice that they can pay later.
I also need an option for the customer to ask for a tender (so that they are able to get discounts for a larger order).

How can I implement this in a good way?

Nobody with an idea?

The invoice part I think I can get by using the PaysonFaktura, but how can I get an option for the customer to choose that they want to get an offer on the products that they have chosen?

Is it possible to change the free checkout so that they are able to choose that?

Would appreciate any ideas.