I am tring to add an international shipping method. But the Store USPS plugin uses 'API' => 'RateV4' Which is for domestic shipping. 'API' => 'RateV2' is for International Shipping. Do I need to just change some values in the default USPS Shipping plugin to make this work?
USPS has separated the International API from Domestic API (Docs) so you would need a new shipping plugin to calculate international rates. In the meantime you can use UPS, FedEx or the Default Shipping Plugin for international orders.