Introduce "zero priced order" concept with no required payment

This applies to cases where an order contains items with zero price or where applied vouchers have reduced the order's total to zero.

app/models/spree/order.rb

@@ -23,7 +23,7 @@ module Spree
       go_to_state :delivery
       go_to_state :payment, if: ->(order) {
         order.update_totals
-        order.payment_required?
+        order.payment_required? || order.zero_priced_order?
       }
       go_to_state :confirmation
       go_to_state :complete
@@ -219,6 +219,12 @@ module Spree
       total.to_f > 0.0 && !skip_payment_for_subscription?
     end
 
+    # There are items present in the order, but either the items have zero price,
+    # or the order's total has been modified (maybe discounted) to zero.
+    def zero_priced_order?
+      valid? && line_items.count.positive? && total.zero?
+    end
+
     # Returns the relevant zone (if any) to be used for taxation purposes.
     # Uses default tax zone unless there is a specific match
     def tax_zone

app/views/spree/shared/_order_details.html.haml

@@ -11,11 +11,15 @@
         %strong
           = order.display_total.to_html
     .pad
-      .text-big
-        = t :order_payment
-        %strong=  last_payment_method(order)&.name
-      %p.text-small.text-skinny.pre-line.word-wrap
-        %em= last_payment_method(order)&.description
+      - if (order_payment_method = last_payment_method(order))
+        .text-big
+          = t :order_payment
+          %strong= order_payment_method&.name
+        %p.text-small.text-skinny.pre-line.word-wrap
+          %em= order_payment_method&.description
+      - else
+        .text-big
+          = t(:no_payment_required)
 
     .order-summary.text-small
       %strong

config/locales/en.yml

@@ -2120,6 +2120,7 @@ en:
   order_not_paid: NOT PAID
   order_total: Total order
   order_payment: "Paying via:"
+  no_payment_required: "No payment required"
   order_billing_address: Billing address
   order_delivery_on: Delivery on
   order_delivery_address: Delivery address