New payment method of Invoicing

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zc - 1.5.5.f - php 5.6

Is there a way to add a new payment systems to modules/payment. We need the customers to simply add items to cart and then we will call them and quote them shipping and take payment via a custom link to our gateway provider. I know this is the complicated way to do this but this is what the client wants. So we need ZC to not calculate taxes or shipping, simple place the order.

Install a Purchase Order payment module. I use the one included with Super Orders. Super Orders is not ready for 1.5.5, but the PO payment module included with it works fine.