Serialize PosAppCartItem.value as decimal instead of string

BTCPayServer.Tests/FastTests.cs

@@ -1162,6 +1162,14 @@ namespace BTCPayServer.Tests
                 }
             }.ToString();
             Assert.Equal(1.65m, JsonConvert.DeserializeObject<PosAppCartItem>(data).Price);
+            data = new JObject()
+            {
+                ["price"] = new JObject()
+                {
+                    ["value"] = "1.6305"
+                }
+            }.ToString();
+            Assert.Equal(1.6305m, JsonConvert.DeserializeObject<PosAppCartItem>(data).Price);
 
             data = new JObject()
             {

BTCPayServer/Services/Invoices/PosAppData.cs

@@ -85,13 +85,7 @@ public class PosAppCartItemPriceJsonConverter : JsonConverter
 
     public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
     {
-        switch (value)
-        {
-            case null:
-                break;
-            case decimal x:
-                writer.WriteValue(x.ToString(CultureInfo.InvariantCulture));
-                break;
-        }
+        if (value is decimal x)
+            writer.WriteValue(x);
     }
 }