I know it's been a while but if you're still looking for a solution try this.

Download and install the 'invoices' module from http://www.zen-cart.com/index.php?ma...roducts_id=230

Create a group called invoice - you can have as many groups as required as long as they're prefixed by invoice_.

Put the customer who can pay on account into this group, they will be offered the 'pay on invoice' option at checkout whereas users not in this group will only get the credit card option.

The nice things about this payment module is it links in with the group discount option. So if you create a group called invoice_3% and have this automatically deduct 3% at checkout the discount will be applied to any memberts of this group.

Cheers,
Q