With Shiptheory, there are a number of ways to ensure that your IOSS number is sent correctly to your carriers as needed. If you're sending orders via the Shiptheory API, it's possible to specify the IOSS in each request. This can be useful if there are multiple IOSS numbers that you use.
To do this, you'll want to ensure you specify the "duty_tax_number_type" and "duty_tax_number" in the Shipment Detail array in your request:
As you can see, in the "duty_tax_number" you'll want to specify your IOSS number, and in the "duty_tax_number_type" you'll want to specify "IOSS". Of course, this is unless you are using one of the other tax number types listed above. It's worth noting that if you already have an IOSS filled out in your Shiptheory account company settings; this number specified in your request will take priority and be used instead.
For more information on this, please see here: https://shiptheory.com/developer/
For some other information on the IOSS in relation to Shiptheory, please see here: https://shiptheory.com/blog/how-to-add-your-ioss-number-to-your-account/
If you have any questions or issues, please feel free to Contact Support