Only level 0 (admin) is able to create metafields.
You need to send the user_id, so the API knows which user is associated to the cart_id
Any metafield key that starts with the prefix
extra_value_will be subtracted or added to the total amount of the cart if the value is an
If you need more information related to Metafields Extra attributes, please click on the link below.
Extra attributes (Metafields)
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