Providing your logistics partner Shiptheory the IOSS number correctly ensures your shipments are handled appropriately.
How to Add IOSS Number to Your Account
Shiptheory will automatically pass your IOSS number directly to your carriers on your behalf.
But in order to do so, you need to add your IOSS number to your account.
There are several ways to do that:
Import your IOSS number manually
Follow Shiptheory's help centre article https://support.shiptheory.com/support/solutions/articles/24000068628-how-to-add-your-ioss-number-to-shiptheory
Using Shiptheory's API
Alternatively, if you are developing with the Shiptheory API, there is the option to send your IOSS number (or any other tax number type) through it using the following parameters:
duty_tax_number:Your IOSS number
duty_tax_number_type:IOSS