feat: handle negative state tax

lib/sales_tax.js

@@ -498,18 +498,29 @@ SalesTax.prototype.__buildSalesTaxContext = function(
   var taxDetails = [];
 
   if (isExempt !== true) {
-    if (countryTax.rate > 0) {
+    // if country has tax and no negative state rate (e.g. FR)
+    if (countryTax.rate > 0 && (!stateTax.rate || stateTax.rate > 0)) {
       taxDetails.push({
         type : countryTax.type,
         rate : countryTax.rate
       });
     }
-    if (stateTax.rate > 0) {
+
+    // if state has tax (e.g. US-HI)
+    if (stateTax.rate && stateTax.rate > 0) {
       taxDetails.push({
         type : stateTax.type,
         rate : stateTax.rate
       });
     }
+
+    // if state has a negative tax, substract it from country's tax (e.g. PT-20)
+    if (stateTax.rate && stateTax.rate < 0) {
+      taxDetails.push({
+        type : stateTax.type,
+        rate : countryTax.rate + stateTax.rate
+      });
+    }
   }
 
   // Build sales tax context

res/sales_tax_rates.json

@@ -502,7 +502,17 @@
 
   "PT": {
     "type": "vat",
-    "rate": 0.23
+    "rate": 0.23,
+    "states": {
+      "20": {
+        "rate": -0.05,
+        "type": "vat"
+      },
+      "30": {
+        "rate": -0.01,
+        "type": "vat"
+      }
+    }
   },
 
   "PR": {

test/sales_tax-test.js

@@ -843,6 +843,47 @@ describe("node-sales-tax", function() {
           );
         });
     });
+
+    it("🇵🇹 should succeed acquiring Açores state sales tax [no tax origin]", function() {
+      return SalesTax.getSalesTax("PT","20")
+        .then(function(tax) {
+          assert.equal(
+            tax.type, "vat", "Tax type should be VAT"
+          );
+
+          assert.equal(
+            tax.rate, 0.18, "Tax rate should be 18%"
+          );
+
+          assert.equal(
+            tax.area, "worldwide", "Tax area should be WORLDWIDE"
+          );
+
+          assert.equal(
+            tax.exchange, "consumer", "Tax exchange should be CONSUMER"
+          );
+
+          assert.equal(
+            tax.charge.direct, true, "Should perform a direct charge"
+          );
+
+          assert.equal(
+            tax.charge.reverse, false, "Should not perform a reverse charge"
+          );
+
+          assert.equal(
+            tax.details.length, 1, "Tax details should contain 1 tax rate"
+          );
+
+          assert.equal(
+            tax.details[0].type, "vat", "Tax details #1 type should be VAT"
+          );
+
+          assert.equal(
+            tax.details[0].rate, 0.18, "Tax details #1 rate should be 18%"
+          );
+        });
+    });
   });
 
   describe("getAmountWithSalesTax", function() {