Sales - Default Payment Terms - another one "End of Month plus box for days"

Sales - Default Payment Terms - another one “End of Month plus box for days”. A client has terms that specify 45 days from the end of the month which the invoice is issued. so the current three choices do not cover this scenario;
sales-default-payment-terms
The invoices issed to this client require an adjustment to the payment due date to fit in with their terms, adding this extra choice would be a useful feature.