FedEx do not support some currency codes, if that is the case we will default to your FedEx default currency set in your Shiptheory FedEx advanced settings.
For a full list of FedEx supported currency codes, you can find that here: https://developer.fedex.com/api/en-jp/guides/api-reference.html#currencycodes
If you encounter any issues, please feel free to contact our support team. You can call us on 0117 403 4313 (UK) / +1 (629) 666-6726 (US) or reach us online here.