Merge pull request #1255 from stripe/remi-invoice-item-amount-optional

Amount for invoice item is now optional

src/Stripe.net/Services/InvoiceItems/StripeInvoiceItemCreateOptions.cs

@@ -6,7 +6,7 @@
     public class StripeInvoiceItemCreateOptions : StripeBaseOptions, ISupportMetadata
     {
         [JsonProperty("amount")]
-        public int Amount { get; set; }
+        public int? Amount { get; set; }
 
         [JsonProperty("currency")]
         public string Currency { get; set; }