Allow for Checkout without payment

Wondering if there is a way to add a payment option of “Bill later” to checkout area? There are certain discounts we are offering (prepaid, longer term, % off when buying something else on the site or existing customer) that can’t seem to be incorporated into GD. So we are thinking easiest way is to allow customer to build their listing, submit and choose a “Contact us for discounted pricing” option so admin can reach out and handle. We are trying to work around inability to offer 4 plans with various term discounts (only option is to create 16 plans).

Any suggestions would be appreciated.

Thanks!