Can I send letters Internationally?

Yes. This article explains how this can be done.

To send International letters with Postworks, the address format must comply with Royal Mail's International addressing guidelines - i.e. the destination country must be the last line of the address. 

Once your address is Royal Mail compliant, simply upload your letter as normal. Postworks' system will then be able to automatically identify the item as an International letter and charge it accordingly.