Set "Proforma only" Payment terms

It would be useful to be able to classify a customer as “Proforma Only” payment terms, i.e. dispatch blocked unless the order has already been marked as paid. Currently I’m setting a customer to having a £0 credit limit which is a bit clunky.

Thanks for the suggestion, Harry. Sorry if I’m missing something obvious, but what is the difference between someone marked a “Proforma Only” and someone with no allowed credit (a credit limit of £0)? Thanks.