Address Analyzer Warnings (#2078)

* Address potential errors

* Add tests

* Add clarity

* Run formatting

test/Core.Test/Models/Business/BillingInfo.cs

@@ -0,0 +1,23 @@
+using Bit.Core.Models.Business;
+using Xunit;
+
+namespace Bit.Core.Test.Models.Business
+{
+    public class BillingInfoTests
+    {
+        [Fact]
+        public void BillingInvoice_Amount_ShouldComeFrom_InvoiceTotal()
+        {
+            var invoice = new Stripe.Invoice
+            {
+                AmountDue = 1000,
+                Total = 2000,
+            };
+
+            var billingInvoice = new BillingInfo.BillingInvoice(invoice);
+
+            // Should have been set from Total
+            Assert.Equal(20M, billingInvoice.Amount);
+        }
+    }
+}