Yes I know that no prices are coded in the extension. But when I have a 9x12 envelope that weighs 10 oz., then First Class International Flats should display when I have it enabled (I'm speaking of my current shopping cart now) This is what their support department told me today:
I've examined USPS shipping services and here's what I've found out.
With this setting:
International shipment mail type -> Envelope
This service is returned for international delivery (i.e. shipment from US to, e.g., England):
U.S.P.S. First-Class Mail® International Large Envelope
I researched through the USPS documentation. There's no exact and verified list of all supported services. And it looks like Large Envelope equals to Flats as there're references of this kind:
First-Class Mail International® Large Envelopes (Flats)
Could you contact USPS support and ask them whether First-Class Mail International Large Envelope and First-Class Mail International Flats are the same services?
Can you confirm that your USPS extension displays the First Class International Flats option? Because this issue is a result of the USPS's recent changes.