Express1 has now added support for USPS First Class Package International. You save 1% on international first class packages (letters and flats are not included).
If you use Teapplix + Express1, your first class international package will automatically print via Express1. To start saving money, open an Express1 account if you have not already:
in Setup => Shipping, set "Discount USPS Provider" to Express1
Save
Go to Setup => Express1, and create an account there. There is no monthly fee and you pay postage via credit card.