Within the Shiptheory shipping rules, it's now possible to have the IOSS number be updated by the rule itself. This is done in the same settings page as the "Update Shipment Details" functionality, more information on that can be found here.
This can be useful for when you may need to send different tax numbers depending on the rule used, rather than one IOSS number for all shipments.
To set this up within a rule, you'll want to edit the rule firstly, and then hit "advanced settings" in the bottom left corner:
From there, you can find the option to update the Duty Tax Type and Number:
Duty Tax Type-
This option lets you specify which tax type the order is going to use, be it IOSS, OSS or any of the other options shown there:
Duty Tax Number-
Now that you've specified the type of tax in the order, you'll now want to specify the number needing to be sent. For example, if using IOSS, you'll need to put the needed IOSS number here:
Once that's all saved, any orders that match that rule will update the Duty Tax Type and Number to what you've specified there.
If you have any issues or questions, please feel free to Contact Support.