2 files changed, 103 insertions, 4 deletions

diff --git a/lib/super_good/solidus_taxjar/tax_calculator.rb b/lib/super_good/solidus_taxjar/tax_calculator.rb
index e498da0..30279bf 100644
--- a/lib/super_good/solidus_taxjar/tax_calculator.rb
+++ b/lib/super_good/solidus_taxjar/tax_calculator.rb
@@ -17,7 +17,7 @@ module SuperGood
         Spree::Tax::OrderTax.new(
           order_id: order.id,
           line_item_taxes: line_item_taxes,
-          shipment_taxes: []
+          shipment_taxes: shipment_taxes
         )
       end
 
@@ -37,6 +37,46 @@ module SuperGood
         end
       end
 
+      def shipment_taxes
+        @shipment_taxes ||=
+          begin
+            # Distribute shipping tax across shipments:
+            # TaxJar does not provide a breakdown, so we have to proportionally
+            # distribute the tax across the shipments, accounting for rounding
+            # errors.
+            total_shipping_tax = taxjar_tax.shipping
+
+            tax_items = []
+            remaining_tax = total_shipping_tax
+            shipments = order.shipments.to_a
+            total_shipping_cost = shipments.sum(&:total_before_tax)
+
+            shipments[0...-1].each do |shipment|
+              percentage_of_tax = shipment.total_before_tax / total_shipping_cost
+              shipping_tax = (percentage_of_tax * total_shipping_tax).round(2)
+              remaining_tax -= shipping_tax
+
+              tax_items << ::Spree::Tax::ItemTax.new(
+                item_id: shipment.id,
+                label: "Sales Tax",
+                tax_rate: tax_rate,
+                amount: shipping_tax,
+                included_in_price: false
+              )
+            end
+
+            tax_items << ::Spree::Tax::ItemTax.new(
+              item_id: shipments.last.id,
+              label: "Sales Tax",
+              tax_rate: tax_rate,
+              amount: remaining_tax,
+              included_in_price: false
+            )
+
+            tax_items
+          end
+      end
+
       def taxjar_breakdown
         @taxjar_breakdown ||= taxjar_tax.breakdown
       end
diff --git a/spec/super_good/solidus_taxjar/tax_calculator_spec.rb b/spec/super_good/solidus_taxjar/tax_calculator_spec.rb
index 02140da..19841a8 100644
--- a/spec/super_good/solidus_taxjar/tax_calculator_spec.rb
+++ b/spec/super_good/solidus_taxjar/tax_calculator_spec.rb
@@ -14,7 +14,40 @@ RSpec.describe ::SuperGood::SolidusTaxJar::TaxCalculator do
       ::Spree::Order.new(
         id: 10,
         store: store,
-        ship_address: address
+        ship_address: address,
+        shipments: [
+          boring_shipment,
+          shipment_with_adjustment,
+          shipment_with_existing_tax
+        ]
+      )
+    end
+
+    let(:boring_shipment) do
+      ::Spree::Shipment.new(id: 1, cost: 7)
+    end
+
+    let(:shipment_with_adjustment) do
+      ::Spree::Shipment.new(
+        id: 2,
+        cost: 20,
+        adjustments: [
+          ::Spree::Adjustment.new(amount: -7)
+        ]
+      )
+    end
+
+    let(:shipment_with_existing_tax) do
+      ::Spree::Shipment.new(
+        id: 3,
+        cost: 10,
+        additional_tax_total: 3,
+        adjustments: [
+          ::Spree::Adjustment.new(
+            amount: 3,
+            source_type: "Spree::TaxRate"
+          )
+        ]
       )
     end
 
@@ -43,7 +76,11 @@ RSpec.describe ::SuperGood::SolidusTaxJar::TaxCalculator do
 
       before do
         allow(dummy_api).to receive(:tax_for).with(order).and_return(
-          instance_double(::Taxjar::Tax, breakdown: breakdown)
+          instance_double(
+            ::Taxjar::Tax,
+            breakdown: breakdown,
+            shipping: 10.0
+          )
         )
       end
 
@@ -66,7 +103,6 @@ RSpec.describe ::SuperGood::SolidusTaxJar::TaxCalculator do
 
         it "returns the taxes" do
           expect(subject.order_id).to eq order.id
-          expect(subject.shipment_taxes).to be_empty
           expect(subject.line_item_taxes.length).to eq 1
 
           item_tax = subject.line_item_taxes.first
@@ -77,6 +113,29 @@ RSpec.describe ::SuperGood::SolidusTaxJar::TaxCalculator do
             expect(item_tax.amount).to eq 6.66
             expect(item_tax.included_in_price).to eq false
           end
+
+          shipment_taxes = subject.shipment_taxes
+          expect(shipment_taxes.length).to eq 3
+
+          aggregate_failures do
+            expect(shipment_taxes[0].item_id).to eq 1
+            expect(shipment_taxes[0].label).to eq "Sales Tax"
+            expect(shipment_taxes[0].tax_rate).to eq tax_rate
+            expect(shipment_taxes[0].amount).to eq 2.33
+            expect(shipment_taxes[0].included_in_price).to eq false
+
+            expect(shipment_taxes[1].item_id).to eq 2
+            expect(shipment_taxes[1].label).to eq "Sales Tax"
+            expect(shipment_taxes[1].tax_rate).to eq tax_rate
+            expect(shipment_taxes[1].amount).to eq 4.33
+            expect(shipment_taxes[1].included_in_price).to eq false
+
+            expect(shipment_taxes[2].item_id).to eq 3
+            expect(shipment_taxes[2].label).to eq "Sales Tax"
+            expect(shipment_taxes[2].tax_rate).to eq tax_rate
+            expect(shipment_taxes[2].amount).to eq 3.34
+            expect(shipment_taxes[2].included_in_price).to eq false
+          end
         end
       end