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Sync: Order & Order Items custom fields
The custom_field fieldtype is a special fieldtype that can read out the custom fields of a synced item. It will render a table in the Channel Entry view and has a tagpair to render the custom_fields in your HTML.
You should use
Fieldtype Tag pair
{order_custom_fields}
{custom_fields:label}
{custom_fields:value}
{/order_custom_fields}