Pirate Ship only offers postage for USPS® parcel shipping services. If you're just trying to mail a letter, you're best off buying some cool stamps at the Post Office 😉
Pirate Ship is built for ecommerce shipping, so we do not offer any USPS® mail services for domestic or international, as those services are meant for letters and documents, not merchandise. Mailing services also do not include tracking (although the labels do include a number that LOOKS like a tracking number, it isn't one), while every shipping service includes tracking.
Since mailing services like First Class Mail International Large Envelope / Flat service are not intended for merchandise, USPS® will reject and return your packages if you try to use these services to mail anything other than documents. You should be using First Class Package® or First Class Package International® instead.