All monetary amounts imported into your ShipStation account are treated as the native currency of your account's home country.
For accounts based in the UK, all monetary values imported into ShipStation from your selling channels and carriers, or the values displayed on your customs declarations, are treated as GBP.
Keep in mind, ShipStation will not do any currency conversion. ShipStation only displays the integers transmitted by the selling channel. The values are assumed to be in the native currency value of your account's origin country. If currency conversion is needed, it will need to be done prior to the orders importing into ShipStation.
If you are uncertain that your selling channel offers this option, please contact your selling channel's support team directly.
Using the native currency value is the best way to ensure that the shipping charges and customs fees/taxes are charged at the appropriate rates!
For any additional support on this, reach out to ShipStation support!