Most pressing is the fact that two of the trade customers whom I usually bill all at once cannot be invoiced for some reason.
By this I mean that I cannot select Invoice in the payment information section of the order total tab.
I thought it was because some of the items were out of stock or on back order, but I've made sure everything is available, and it still won't allow me to select invoice.

The second part of this issue is that I don't know how to set up a customer so that I have the option to select invoice instead of credit card payment/cheque or whatever
Thanks in advance!