Profile: Support

  <FeedAttribute>
    <Id>31</Id>
    <FeedCaption>Custom label for Shopping Campaigns 0</FeedCaption>
    <FeedName>custom_label_0</FeedName>
    <FieldNameCode>productcost</FieldNameCode>
    <ConvertValues />
    <CharsLimit>100</CharsLimit>
    <Required>false</Required>
    <IgnoreNullOrEmpty>false</IgnoreNullOrEmpty>
    <UseCDATA>false</UseCDATA>
    <ActionId>0</ActionId>
    <AdvancedField>true</AdvancedField>
    <RowSourceType>0</RowSourceType>
    <AttributeControlType>0</AttributeControlType>
  </FeedAttribute>

ID is unique key for each field.
Change it with value 45645654 and try.

Check if you has
<Id>51</Id>
only one time.
Maybe you copy existing ID and this template has two items with ID 51

I'll add them.
Can you send  me link for documentation?
Or send me example?

About discount code and discount amount.
Order can have several discount codes. I need to see an example too.

Here you can order a new plug-in functionality.

How to install and configure the module in the online store.

or read our documentation link

Here you can discuss plug-in operation.

Topic for reporting any problems and bugs.

IMPORTANT NOTE: There is a known issues with installing new plugins in nopCommerce.
Please read this post for more information.

Why didn't you use the city for billing address?
You use only city for shipping address.
Example

"billing_address":{"country_code":"IT","postal_code":"20101","state":"MI"}}
"shipping_address":{"recipient_name":" ","city":"Milano","country_code":"IT","postal_code":"20101","state":"MI"}

Thank you too.
Please write a review about our support
https://www.nopcommerce.com/p/2707/tracking-code-manager-facebook-pixel-google-tag-manager-tradetracker-etc-foxnetsoftcom.aspx