One of the technical requirements for shipping internationally for some carriers is that all products being sent have a value greater than 0. This proves difficult when sending samples, or free gift products as part of a bigger order containing products.

To solve this problem when shipping internationally from Magento 2, you can set Shiptheory to fallback to the cost price of the product. Assuming you have set cost prices for your products in your Magento catalogue, you just need to browse to your advanced Magento 2 settings in Shiptheory and enable the "Fallback to Product Cost Price" option, shown below.