if iseligible is false, do not allow calculator to hit api

app/models/spree/calculator/avalara_transaction.rb

@@ -13,6 +13,7 @@ def compute_shipment_or_line_item(item)
       item_address = order.ship_address || order.billing_address
       prev_tax_amount = prev_tax_amount(item)
 
+      return prev_tax_amount unless Spree::AvalaraPreference.iseligible.is_true?
       return prev_tax_amount if %w(address cart).include?(order.state)
       return prev_tax_amount if item_address.nil?
       return prev_tax_amount unless calculable.zone.include?(item_address)

spec/models/spree/calculator/avalara_transaction_spec.rb

@@ -63,18 +63,25 @@
           expect(calculator.compute(line_item)).to eq(0.4)
         end
 
-        it 'should be equal to the previous additional_tax_total is order is at cart' do
+        it 'should be equal to the previous additional_tax_total if order is at cart' do
           order.state = 'cart'
           line_item.additional_tax_total = 0.1
           expect(calculator.compute(line_item)).to eq(0.1)
         end
 
-        it 'should be equal to the previous additional_tax_total is order is at address' do
+        it 'should be equal to the previous additional_tax_total if order is at address' do
           order.state = 'address'
           line_item.additional_tax_total = 0.1
           expect(calculator.compute(line_item)).to eq(0.1)
         end
 
+        it 'should be equal to the previous tax total if preference iseligible is false' do
+          Spree::AvalaraPreference.iseligible.update_attributes(value: 'false')
+
+          line_item.additional_tax_total = 0.1
+          expect(calculator.compute(line_item)).to eq(0.1)
+        end
+
         context 'when the order is discounted' do
           let(:promotion) { create(:promotion, :with_order_adjustment) }