When creating Shipping Rules within your Shiptheory account, you may want to create certain rules IOSS-applicable EU countries which require a certain shipping service.
The following is the list of countries to which IOSS should apply for use with your rules:
Austria
Belgium
Bulgaria
Croatia
Republic of Cyprus
Czech Republic
Denmark
Estonia
Finland
France
Germany
Greece
Hungary
Ireland
Italy
Latvia
Lithuania
Luxembourg
Malta
Netherlands
Poland
Portugal
Romania
Slovakia
Slovenia
Spain
Sweden
If you're looking for how to configure IOSS specific to a carrier, this would be a good place to start: IOSS Support. There are a number of carrier-specific guides for configuring IOSS shipping rules, including a guide for Royal Mail and for DHL.
If you have any questions not covered here, please don't hesitate to contact support here!