I created the ability for users to be able to get an invoice with the payment system using the Payment on Invoice from the pre-defined list, but it just seems so natural that I should be able select the invoice option from the Call Center on the back end. The problem is that in order for the Call Center to use the Invoice payment system it must be an Advanced system. Why is this? It seems that if it works from the front end of the website I should be able to enable this on the back end. What can I do to enable this?

You can set the invoice payment system as advanced if you create an empty file in the folder payments named "invoice_payments.php" (or something like it) and then refer to it in the "Advanced library" field and "Advanced url" field. (payments/invoice_payments.php). Then you can choose Invoice payment system when making a call center order.

Thanks for the tip. Works, but the problem is that you can only have one payment system configured for the call center. What if I need to take credit cards or invoice?

I know, that has bothered me as well. But I solved it like this: when that happens I change the call center payment system right before I make the new call center order. And then I change it back again.

To solve this problem we created a custom payment field that was two radio buttons: Credit Card and PO (we added more later). Then, in our payment system we added to code to check and see if PO was set, and if so, we skipped the credit card processing and marked it as valid.

Gustaf, would you be willing to share your custom payment system with me?