If the nested array is being sent from a server action, i.e. not from a browser action, then your options are limited to either:
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Build up the string or parts of it before the API call, and store it ready to send.
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For example, as each price item is created or updated, create a string of the JSON object { amount, currency, tax }. Then all the strings can be retrieved in a list.
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Another example is to use the List Item Expression to build up the whole list from a Browser action, and store it for use by the server action.
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If the list contents depends on server-side actions, you could call an external service to help build up the string.
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Possibly build up the string by running an API workflow on a list, I don’t particularly like this option so I haven’t tried it.